Strawberry tree jam yield calculator: jars per kilo, exactly

This calculator turns a weight of fresh Arbutus unedo fruit into a finished jam batch: how much pulp you recover, how much sugar to add, how many jars you fill, and what each jar costs to make. It is aimed at home preservers working through a garden glut and at smallholders running a small value-added line off their own trees.

Strawberry tree jam behaves differently from strawberry or apricot jam because the fruit is low in juice and thick with stone cells, so a kilo of fresh berries yields less usable pulp than you would expect. Guessing the sugar and jar count from a strawberry recipe leaves you short on jars or long on runny jam.

Enter the fresh weight and a few recipe choices and the tool works forward through pulping, sugaring, boiling and filling, printing a jar count and a cost per jar you can price against. Every number rests on a recovery fraction and an evaporation loss you can override once you have run a batch of your own.

🌱 How to use the strawberry tree jam yield calculator

Begin with fresh fruit weight in kilograms, the anchor for everything downstream. The default is 2 kg, a typical single-tree pick. The tool assumes you have already removed stalks and obviously mouldy berries, so weigh the fruit you actually intend to cook.

The fruit-to-sugar ratio comes next, chosen as a proportion of sugar to prepared pulp. Strawberry tree pulp is mild and low in acid, so the reconstructed default is a 1 to 0.7 ratio, meaning 700 g of sugar per kilo of pulp. Sweeter setups push toward 1 to 1, which also improves the set and shelf stability.

Pectin type follows. Strawberry tree fruit is low in natural pectin, so the tool offers none, added powder and high-methoxyl liquid options. The choice shifts the set factor and the amount of sugar the recipe needs to gel, and it changes how long you boil.

Because Arbutus unedo is naturally low in pectin and acid, most reliable strawberry tree jam recipes add both pectin and a splash of lemon juice. The tool assumes lemon is present when you select an added-pectin option.

Target set lets you pick soft, standard or firm. A softer set keeps more water and yields slightly more finished jam per kilo, while a firm set boils longer, loses more water and yields fewer but denser jars. The default is standard.

The cooking loss adjustment is a percentage you can nudge if your stove runs hot or your pan is wide. The default evaporation loss is 15 percent of the sugared mass, and a wide shallow pan on a fierce burner can push that past 20. Raise it if your past batches came out thicker than the tool predicted.

Finally, jar size in millilitres sets how the finished weight splits into containers. The default is 340 ml, a common preserving jar. As you change fruit weight or set firmness, the jar count and cost per jar update together, so you can size a batch to the jars you actually own.

🌱 Calculator fields explained

Fresh fruit weight (kg) – The de-stalked, sorted weight of berries going into the pan, default 2 kg. Everything scales from this figure, so weigh after cleaning, not before.

Fruit-to-sugar ratio – The proportion of sugar to prepared pulp, default 1 to 0.7 (700 g sugar per kg pulp). Higher sugar improves set and shelf life but makes a sweeter jam. Lower sugar needs added pectin to gel.

Pectin type – None, added powder, or high-methoxyl liquid. Default is added powder, since strawberry tree fruit sets poorly on its own. The choice adjusts the set factor and boil time.

Target set (soft/standard/firm) – How stiff the finished jam is, default standard. Firmer sets boil longer and yield fewer jars from the same fruit.

Cooking loss adjustment (%) – The evaporation loss during boiling, default 15 percent. Raise it for wide pans and high heat, lower it for a deep narrow pan on a gentle simmer.

Jar size (ml) – The volume of each jar, default 340 ml. Sets how the finished jam divides into jars and drives the cost-per-jar figure.

🌱 Understanding the results

The calculator reports six figures that trace the batch from raw fruit to priced jar. Read them in sequence, because each feeds the next.

The hero number is the jar count, since it is what you plan the day around. Prepared pulp weight is the first secondary number and the one most growers underestimate: the reconstructed recovery fraction is 0.55, so 2 kg of fresh fruit yields about 1.1 kg of usable pulp after the seeds and skin grit are sieved out.

Total sugar follows directly from pulp and the ratio. At the default 0.7, that 1.1 kg of pulp takes about 770 g of sugar, giving a sugared mass near 1.87 kg before boiling.

Finished jam weight is where evaporation bites. At the default 15 percent loss, the sugared mass drops to roughly 1.59 kg, which at 340 ml jars filling around 450 g each gives about three and a half jars.

Sugar concentration below 60 Brix means the jam is not shelf-stable at room temperature and must be refrigerated and eaten within weeks. If the tool reports under 60, raise the sugar ratio or boil longer before you count on a pantry life.

Cost per jar rests on three ingredient prices in the parameter table: fruit, sugar and the empty jar. For home growers the fruit cost is often zero, which drops the cost per jar sharply and is worth setting deliberately rather than accepting a market fruit price you did not pay.

The Brix output is your safety check. Set jam needs about 65 Brix to keep in the pantry unopened. A soft set with low sugar can land near 60, which is fine for the fridge but not for a market stall selling ambient-stored jars.

Reading the extremes: a firm set on a wide pan can push evaporation loss past 20 percent and cut a jar off the count, while a soft low-sugar set yields more jars that must be refrigerated. Neither is wrong, but they route to different storage and price.

🌱 Calculation formulas

The tool chains five steps: recover pulp, add sugar, apply the set and pectin adjustment, evaporate, then divide into jars. The core expressions use the reconstructed constants below.

pulp = fresh_weight × 0.55

sugar = pulp × sugar_ratio

sugared_mass = pulp + sugar

finished = sugared_mass × (1 − loss% ) × set_factor

jars = finished ÷ jar_fill_weight

Brix of the finished jam is estimated from the sugar fraction of the boiled mass: Brix ≈ (sugar + pulp_solids) ÷ finished × 100, where pulp solids are taken as about 0.12 of the pulp weight.

The set factor and pectin adjustment are small multipliers, rarely more than 3 percent either way. They matter at the margin of a jar count, not to the overall scale of a batch, so do not chase them at the expense of a proper boil test.

Full walkthrough at defaults: fresh weight 2 kg gives pulp 1.1 kg. Standard sugar ratio 0.85 gives sugar 0.935 kg, so sugared mass is 2.035 kg. Standard loss 15 percent and set factor 1.00 with powder pectin 1.00 give finished 2.035 × 0.85 × 1.00 × 1.00, about 1.73 kg. Divided by 0.45 kg per jar, that is roughly 3.8 jars, rounded down to 3 full jars with a part jar over.

🌱 Practical examples

Each scenario shows inputs, calculation and the next move. Ingredient costs use the parameter table unless the grower supplies their own.

1. Single tree, home preserver. Inputs: 2 kg, standard, powder pectin, 340 ml jars, home-grown fruit (cost 0). Pulp 1.1 kg, sugar 0.935 kg, finished 1.73 kg, about 3 jars. Cost per jar covers only sugar and jar, near 0.90. Next: label three jars and keep one part-jar in the fridge.

2. Two-tree glut. Inputs: 4 kg, standard, powder, 340 ml. Pulp 2.2 kg, sugar 1.87 kg, finished 3.46 kg, about 7 jars. Next: scale lemon juice to taste and boil in two pans to keep the set consistent.

3. Soft set for a soft-fruit lover. Inputs: 2 kg, soft, powder, 340 ml. Sugar ratio 0.7, loss 12 percent, set factor 1.02. Pulp 1.1 kg, sugar 0.77 kg, finished about 1.68 kg, close to 3 jars. Brix near 60. Next: refrigerate and eat within a month.

Always run a cold-plate wrinkle test near the end of the boil rather than trusting the predicted jar count alone. The calculator sizes the batch; your plate confirms the set.

4. Firm set for gifting. Inputs: 2 kg, firm, HM liquid, 340 ml. Sugar ratio 1.0, loss 20 percent, set factor 0.97, pectin 1.02. Pulp 1.1 kg, sugar 1.1 kg, finished about 1.7 kg, roughly 3 dense jars at 65 Brix. Next: these keep in the pantry, so label with the date and gift freely.

5. Smallholder trial run. Inputs: 10 kg, standard, powder, 340 ml, fruit at 4.00 per kg. Pulp 5.5 kg, sugar 4.68 kg, finished 8.65 kg, about 19 jars. Ingredient cost near 44.00 for fruit, 5.60 sugar, 11.40 jars, so cost per jar around 3.20. Next: price at least at 6.00 to cover labour and stall fees.

6. Smallholder using free windfall fruit. Inputs: 10 kg, standard, powder, 340 ml, fruit cost 0. Same 19 jars, but ingredient cost drops to sugar and jars only, near 17.00, so cost per jar falls to about 0.90. Next: free fruit cuts the cost per jar from 3.20 to 0.90. Price competitively or bank the margin.

7. Commercial batch to fill a jar order. Inputs: work backwards from 50 jars needed at 340 ml. Finished target about 22.5 kg, so sugared mass near 26.5 kg, requiring pulp about 14.3 kg and fresh fruit around 26 kg. Next: confirm you can pick and pulp 26 kg before the fruit over-softens.

8. Edge case, over-ripe glut with low pectin. Inputs: 3 kg soft splitting fruit, none for pectin, standard set. Pectin factor 0.95 drops the finished mass and the jam sets loose. Next: switch to added powder or accept a syrupy result destined for the fridge.

9. Edge case, wide pan on high heat. Inputs: 2 kg, standard, powder, but cooking loss raised to 25 percent. Finished drops to about 1.53 kg, shaving the count to just under 3 jars. Next: lower the heat or use a deeper pan to recover the lost jar.

🌱 Tips and best practices

Weigh the fruit after de-stalking and sorting, never before. Stalks and cull fruit can be a tenth of a rough pick, and the tool assumes the weight you enter is all going into the pan.

Add lemon juice with any low-sugar or low-pectin setup. Strawberry tree pulp is short on both acid and pectin, and a tablespoon of lemon per kilo of pulp does more for the set than a small change in the set factor.

Sieve the cooked fruit to remove the stone cells that make raw Arbutus unedo gritty. A fine mesh or a food mill turns a grainy pulp into a smooth spread and is the step most first-timers skip.

Run one honest test batch and reweigh at every stage. Your own recovery fraction and evaporation loss will beat any default, and once you have them the tool predicts your kitchen to within half a jar.

Fill jars hot and to a consistent level so the jar count holds. A wandering fill line is the commonest reason a predicted seven jars becomes six with a large gap in the last one.

Match the sugar to the storage you plan. Above 65 Brix the jam keeps unopened in a cool pantry, while a soft 60 Brix jam belongs in the fridge, so decide the storage before you set the ratio.

Boil in batches no larger than your pan handles well. A pan more than two-thirds full spits and scorches, and scorched pulp both tastes bitter and throws off the finished weight.

Price commercial jars on your real cost per jar, not the ingredient cost alone. Labour, fuel, labels and stall fees typically double the tool’s figure, so treat cost per jar as a floor rather than a price.

Keep a wide-mouth funnel and a set of matched jars to hand. Consistent 340 ml jars make the count reliable and let you reuse the tool’s cost figure batch after batch.

⚠️ Common mistakes to avoid

Expecting strawberry-jam yields

Strawberry tree fruit gives roughly half its weight as usable pulp, far less than a strawberry, because seeds and stone cells are sieved away. Planning a batch on a strawberry recipe leaves you short of jars.

Use the 0.55 recovery figure as a starting point and reweigh your own first batch to correct it.

Skipping added pectin

Relying on the fruit’s own pectin gives a syrupy jam that never sets firm. The none option carries a 0.95 penalty for a reason.

A batch that will not set is not just soft, it is unsellable as ambient jam and must be refrigerated, so a skipped spoon of pectin can strand twenty jars in the fridge.

Add powder or liquid pectin and a splash of lemon for any batch you intend to store at room temperature.

Ignoring the Brix warning

A low-sugar soft set can land under 60 Brix, which will not keep in the pantry. Selling those jars as shelf-stable risks spoilage in the customer’s cupboard.

Treat 65 Brix as the target for ambient storage and refrigerate anything softer.

Overfilling the pan

A pan crammed past two-thirds spits, scorches and boils unevenly, which ruins flavour and skews the finished weight the tool predicted.

Split large batches across pans or cook in stages, keeping each boil to a manageable depth.

Using market fruit cost for home-grown fruit

Leaving the default fruit price in place when the fruit came free from your own tree inflates the cost per jar two or three times over. That can talk you out of a perfectly profitable stall.

Charging yourself 4.00 a kilo for fruit you picked for nothing is a bookkeeping fiction that hides your real margin and can kill a good side line before it starts.

Set the fruit cost to zero for home-grown fruit and price on the true sugar-and-jar cost.

Not sieving out the grit

Raw Arbutus unedo pulp is granular with stone cells, and jam made without sieving feels sandy on the tongue. Customers notice immediately.

Pass the cooked fruit through a fine mesh or food mill before adding the bulk of the sugar.

Guessing evaporation instead of measuring

The default 15 percent loss suits a medium pan on a moderate boil, but a fierce wide-pan boil can hit 25 percent and quietly cost you a jar. An unmeasured 10 percent extra loss can erase a full jar per batch.

Weigh the finished jam once and adjust the cooking loss field to match your setup.

🌱 When to use this calculator

Reach for it before a big pick when you need to know whether the fruit on the tree will fill the jars you have, or whether to buy more jars or borrow another pan. That go or no-go on jar count is the decision the tool sharpens most.

Use it when you are costing a small commercial line and need a defensible cost per jar to price against. The ingredient breakdown gives you a floor, and you add labour and overhead on top.

It also helps when you are converting a strawberry or apricot recipe to strawberry tree fruit, since the lower pulp recovery and weaker pectin change the sugar and jar maths in ways the old recipe hides.

The calculator plans the batch; it does not replace the boil test. Treat the jar count as the target and the cold-plate wrinkle as the verdict, and the two together rarely disappoint.

Skip it for a casual half-kilo of fruit you are simmering into a bowl to eat that week. At that scale the maths is trivial and the tool only slows you down.

🌱 Related calculators

• Arbutus ripeness
• Arbutus nutrition
• Arbutus tree yield

🌱 Glossary

Arbutus unedo – The strawberry tree, whose autumn red fruit is low in juice, acid and pectin, giving a granular pulp when cooked.

Prepared pulp – The usable, sieved fruit flesh after seeds and stone cells are removed, here about 55 percent of fresh weight.

Fruit-to-sugar ratio – The proportion of sugar added per unit of pulp, controlling sweetness, set and shelf life.

Pectin – The gelling agent that sets jam. Strawberry tree fruit is low in it, so added powder or liquid pectin is usual.

High-methoxyl pectin – A pectin type that gels with high sugar and acid, suited to standard sugary jams.

Set – The firmness of the finished jam, from soft spread to stiff sliceable, controlled by sugar, pectin and boil time.

Brix – Soluble solids, mostly sugar, as a percentage. Around 65 Brix is the usual target for pantry-stable jam.

Why does my finished jam weigh less than the fruit plus sugar? Because boiling evaporates water. The tool models that loss at 12 to 20 percent depending on your set, which is why the finished weight is always below the sugared mass.

Evaporation loss – The water driven off during boiling, reducing the sugared mass to the finished weight.

Set factor – A small multiplier reflecting how much water a soft, standard or firm set retains.

Sugared mass – The combined weight of pulp and added sugar before boiling begins.

Jar fill weight – The mass of jam a jar holds, about 450 g for a 340 ml jar, used to convert finished weight into a jar count.

Cold-plate test – Dropping a little jam on a chilled plate to see if it wrinkles, the traditional check for a good set.

❓ Frequently asked questions

How much fruit do I need for six jars?

At the defaults, six 340 ml jars need about 2.7 kg of finished jam, which traces back to roughly 3.5 kg of fresh fruit after pulp recovery and boiling loss.

That figure assumes the standard set and 15 percent evaporation. A firm set loses more water and pushes the fruit requirement toward 3.9 kg for the same six jars.

Why is the pulp recovery so low?

Strawberry tree fruit is dense with seeds and stone cells that you sieve out, so only about 55 percent of the fresh weight survives as usable pulp. A juicier fruit like a strawberry recovers far more.

This is the single biggest surprise for first-time strawberry tree preservers, and the main reason a strawberry recipe leaves you short on jars.

Do I have to add pectin?

Not strictly, but without it the jam sets loose and syrupy because Arbutus unedo carries little natural pectin. The none option applies a 0.95 penalty to reflect the weaker set.

For any jam you plan to store at room temperature, add powder or liquid pectin and a little lemon juice so it gels and keeps.

What sugar ratio gives a shelf-stable jam?

Aim for enough sugar to reach about 65 Brix in the finished jam, which usually means a ratio near 0.85 to 1.0 sugar to pulp with a standard or firm set.

The tool reports the Brix so you can check. Anything under 60 should be refrigerated and eaten within weeks rather than stored in the pantry.

Can I use frozen strawberry tree fruit?

Yes. Freezing suits this fruit well and the pulp recovery is similar, though thawed fruit releases more juice, so you may see slightly higher evaporation loss.

Bump the cooking loss field a couple of points if you are working from frozen, and expect the set to need the full pectin dose.

How do I cost jars for selling?

The cost per jar sums fruit, sugar and the empty jar from the parameter table. For a stall, treat that as a floor and add labour, fuel, labels and fees on top.

A tool cost of 3.20 a jar often becomes a 6.00 to 7.00 shelf price once real labour and overhead are counted. The calculator gives the ingredient floor, not the sticker.

Set your fruit cost to zero for home-grown fruit so the floor reflects what you actually spent.

Why does a firmer set give fewer jars?

A firm set boils longer to drive off more water, so the finished weight is lower even though the fruit and sugar going in are the same. Fewer jars, but each is denser and keeps better.

The tool models this with a higher evaporation loss and a set factor below 1.0 for firm jams, trimming the count by up to a jar per batch.

My jam did not set. What went wrong?

The usual causes are too little pectin, too little acid, or too little sugar. Strawberry tree fruit is short on the first two, so a batch made like a strawberry recipe often runs loose.

Reboil with added pectin and a squeeze of lemon, and check the cold-plate test before jarring. A jam that reads under 60 Brix will also stay soft.

Can I halve the sugar for a low-sugar jam?

You can, but you must switch to a low-methoxyl pectin designed to set with less sugar, and you should refrigerate the result. The high-methoxyl options in the tool need the sugar to gel.

A halved-sugar jam will also read well under 60 Brix, so it is a fridge jam with a short life, not a pantry preserve.

How long does the finished jam keep?

A properly set jam above 65 Brix, jarred hot into sterile jars, keeps unopened for about a year in a cool dark pantry and a few weeks once opened and refrigerated.

Softer, lower-sugar batches under 60 Brix keep only a few weeks even unopened and belong in the fridge from the start.

⚖️ Disclaimer

This calculator and article offer educational and agricultural planning information for people preserving strawberry tree fruit. The pulp, sugar, jar and cost figures are estimates for planning a batch, not guarantees of yield or set in any particular kitchen.

Results depend on cultivar, fruit ripeness, pan shape, heat and the accuracy of your own weights, and the reconstructed coefficients here should be checked against a measured test batch. Local prices for fruit, sugar and jars vary widely and change the cost per jar directly.

Any pricing or profit figure you draw from the cost per jar is a commercial decision of your own and not investment advice. It excludes labour, fuel, packaging and market fees, which usually exceed the ingredient cost.

Home preserving carries food-safety responsibilities around sterilisation, sugar concentration and storage. For safe canning practice and shelf-life guidance, follow your local food-safety authority or extension service rather than this planning tool, and consult a professional if you are selling to the public.