What figures do you need to check portion-size drift?
Portion-size drift means the amount served differs from the amount in your costed recipe. Here, assume a purchased, ready-to-use ingredient costs £12.00 for 1,000g. The recipe specifies 50g per dish, but a measured service average is 65g. These are invented arithmetic inputs, not supplier prices or observations from a real kitchen.
Assume all other chosen direct costs total £2.40 per sold dish and the customer price is £9.60, including an explicitly assumed 20% sales VAT. The VAT rate does not classify any particular food sale.
All ingredient weights use the same ready-to-use basis. Assume no trimming loss, cooking loss or unsold portions in this example. Other costs and sales volume stay unchanged. Costs exclude recoverable purchase VAT and include non-recoverable VAT.
The worked numbers were checked using Python 3. Money is shown to pennies and percentages to two decimal places, retaining full precision during calculations.
How do you convert an extra spoonful into a portion cost?
Start with cost per gram: £12.00 ÷ 1,000g = £0.012 per gram.
At the recipe quantity, the ingredient costs 50g × £0.012 = £0.60 per dish. Adding the other £2.40 gives a total entered portion cost of £3.00.
At the measured quantity, it costs 65g × £0.012 = £0.78. Total portion cost becomes £3.18.
The difference is 15g and £0.18 per dish. Use the average measured weight when estimating an average cost; one unusually generous serving cannot establish the usual portion size. If a purchased ingredient needs preparation before weighing, first establish its cost per usable gram rather than mixing purchased and prepared weights.
What happens to GP at the unchanged menu price?
Net sale means the customer price before sales VAT. Gross profit is that net sale minus the entered portion cost. GP margin expresses gross profit as a percentage of the net sale.
Net sale = gross customer price ÷ (1 + VAT percentage ÷ 100). Gross profit = net sale − portion cost. GP margin = gross profit ÷ net sale × 100.
For this example, £9.60 ÷ 1.20 = £8.00 net sale.
With the recipe cost of £3.00, gross profit is £5.00 and GP margin is 62.50%. With the measured cost of £3.18, gross profit is £4.82 and GP margin is 60.25%.
The extra ingredient reduces GP by 18p per sale and margin by 2.25 percentage points. Neither margin is a recommended target. Gross profit still has to cover remaining business expenses; it is not net or take-home profit.
How much does that mean across a trading period?
At an assumed 300 sold portions, the additional ingredient used is 15g × 300 = 4,500g, or 4.5kg. At £12 per kilogram, its cost is £54.00.
The recipe calculation would show 300 × £5.00 = £1,500.00 gross profit. The measured portions instead produce 300 × £4.82 = £1,446.00 against the entered costs.
This £54 difference holds the number of sales, menu price and all other costs constant. It is not a forecast of savings. Changing the serving size could affect the dish and customer response, so check whether the recipe specification is still appropriate before treating the difference as avoidable cost.
How can you check the portion during service?
Weigh several representative servings using a suitable scale, subtracting the container weight. Record the total ingredient weight and divide by the number of servings measured. Include different service periods or people preparing the dish where relevant, and record the sample size rather than presenting a small sample as certainty.
Compare that average with the costed recipe. Decide whether to restore the intended serving method or update the recipe cost to reflect the portion customers actually receive. A measured scoop or a clear gram specification can make the chosen portion easier to reproduce.
Keep portion-size drift separate from unsold food. The calculator’s waste percentage means unsold portions, not extra grams served or cooking yield loss. Do not enter the same cost increase again as a waste allowance.
How do you reproduce the check in GP Calculator UK?
Open GP Calculator UK at / and choose Check my GP. Enter cost 3.00, customer menu price 9.60 and assumed VAT 20. Record the £5.00 gross profit and 62.50% margin. Then change only cost to 3.18 to see £4.82 and 60.25%.
Use your own measured weights, purchase costs and applicable VAT treatment before making a decision. Read /#guide for input help and /methodology/ for calculation assumptions and rounding.
HMRC’s catering guidance explains how VAT treatment depends on the sale: https://www.gov.uk/guidance/catering-takeaway-food-and-vat-notice-7091
Purchase VAT recovery has conditions and exceptions. Use costs excluding recoverable VAT and including non-recoverable VAT; HMRC explains recovery here: https://www.gov.uk/charge-reclaim-record-vat/reclaim-vat-business-expenses
This guide explains costing arithmetic, not personalised tax advice.
Sources
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Use the free GP calculator to check selling prices, compare margins and save menu items on your device. Gross profit is before remaining business expenses.
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