The short answer: yield percentage = (usable weight after prep ÷ purchased weight) × 100. To find the true cost of what you actually serve, divide the price you paid per kilo by the yield percentage (as a decimal). So if a whole salmon costs £12/kg and you get 55% usable fillet, your real cost is £12 ÷ 0.55 = £21.82 per kg of servable fish — not £12. That gap is where most menu costings go wrong.
Below is the full method, worked examples for meat, fish and veg, and how to build your own yield reference so you only have to do the test once per ingredient.
The price on your invoice is the As Purchased (AP) price. The price that actually matters is the Edible Portion (EP) cost — what one usable kilo or gram costs after you've removed skin, bones, peel, sinew, roots, and everything else that ends up in the bin (or the stock pot).
If you cost recipes using AP prices, every ingredient with trim loss is under-costed. A dish that looks like it runs at 28% food cost might really be at 36%, and you'll only find out when the margins don't materialise at the end of the month.
Two ingredients at the same shelf price are almost never the same value. Whole chicken at £4/kg with a 65% yield costs £6.15/kg usable. Pre-portioned breast at £7/kg with a 98% yield costs £7.14/kg usable — a much smaller gap than the shelf price suggests, and that's before you value the carcass for stock.
You need a scale, a knife, and ten minutes. Do this once per ingredient and record the result.
1. Weigh the item as purchased. Whole fish, untrimmed sirloin, unpeeled carrots — exactly as it comes off the delivery.
2. Prep it exactly as you would for service. Don't butcher more carefully than you normally would. The point is to capture your real-world yield, including your kitchen's habits and skill level.
3. Weigh the usable portion. Only what goes on a plate or into the recipe.
4. Optionally, weigh the by-products separately. Bones for stock, trim for mince, fish frames for fumet — these have value and can be credited (more on this below).
5. Calculate:
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Yield % = (usable weight ÷ purchased weight) × 100
EP cost per kg = AP price per kg ÷ (yield % ÷ 100)
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Accuracy here lives and dies by the scale. Bathroom-style guessing won't cut it for a 2.3kg fish — a decent digital scale like the is the single cheapest fix for inconsistent costings, and you'll use it for portioning anyway.
Yield: 3.6 ÷ 4.5 = 80%
EP cost: £14.00 ÷ 0.80 = £17.50/kg
If you cut 250g steaks, each portion costs £4.38 in meat — not the £3.50 the invoice price implies. On a striploin that's an 88p-per-steak error. Sell 40 steaks a week and that's over £1,800 a year of margin you thought you had.
Fish is the most brutal category for yield. Typical whole-fish-to-skinless-fillet yields run 40–55% depending on species and knife skills.
Yield: 0.81 ÷ 1.8 = 45%
EP cost: £11.00 ÷ 0.45 = £24.44/kg
A 160g fillet portion costs £3.91 — more than double what the AP price suggests. This is why filleting your own fish only pays if you're using the frames and trimmings, or if the whole-fish price discount is genuinely large.
Veg yields are gentler but they compound across a menu because you use so much of it.
Yield: 82%. EP cost: £0.90 ÷ 0.82 = £1.10/kg
A 22% uplift on carrots sounds trivial, but apply the same logic to onions (~88%), leeks (~50–60%), celeriac (~75%), and cauliflower (~55–60% for florets only) and your "cheap" veg garnish is often 40–70% more expensive than the invoice says.
If your bones become stock and your trim becomes staff meals or mince, that trim isn't pure waste — it has value. The honest way to handle it:
1. Assign the by-product a fair market value (what you'd pay to buy the equivalent — e.g. fish frames at £1.50/kg).
2. Subtract that value from the total AP cost before dividing by the usable weight.
Using the sea bass: 700g of frames and trim valued at £1.50/kg = £1.05 credit. Adjusted cost = (£19.80 − £1.05) ÷ 0.81 kg = £23.15/kg instead of £24.44. Only take the credit if you genuinely use the by-product — otherwise you're just flattering your numbers.
The whole point of yield testing is that you do it once per ingredient, record the percentage, and apply it automatically every time that ingredient appears in a recipe. Keep a simple table: ingredient, supplier, AP price, yield %, EP cost, date tested. Review quarterly or when prices move.
Doing that maths by hand across a full menu is exactly the kind of repetitive work that quietly stops happening in a busy kitchen — which is when the guessing creeps back in. If you'd rather have it handled for you, try the free food cost calculator at getprepsheet.com to run your EP costs and portion maths in seconds, or grab one of the ready-made Excel costing templates in the shop with yield percentages built in — so every recipe you cost from now on uses the real numbers, not the invoice ones.