Eggs and dairy are costed in the same unit your recipe uses: if it says "3 eggs", you need the price per egg; if it says "170 g butter", the price per gram. The catch is that you almost never buy them that way. Eggs come in trays, butter in blocks, milk in cartons and cheese by the kilo with a part that never reaches the plate. The fix is to convert with your own measurements: count, weigh and write it down.
Eggs: by count or by weight
If you buy a tray and your recipe counts eggs:
price per egg = tray price ÷ eggs in the tray
That works while the eggs stay the same size. When you switch between medium and large, the price per egg changes and so does how much egg goes into the recipe. To compare sizes fairly, compare price per kilo. Weigh a full tray of each on your scale. Say a tray of 30 medium costs $8.10 and weighed 1,650 g, and a tray of 30 large costs $9.00 and weighed 1,860 g. Medium works out at 8.10 ÷ 1.65 = $4.91 a kilo, large at 9.00 ÷ 1.86 = $4.84. Those are example numbers; your eggs will give your own. The unit price tool does this comparison per kilo.
For baking there is a steadier way: write the recipe in grams of egg without shell. To cost it, you need to know how much of what you pay for is shell. Crack a dozen, weigh the contents and compare it with what they weighed whole. If 1,000 g of eggs gives you 880 g without shell, your cost per usable kilo is the price per kilo ÷ 0.88. That is your measurement, with your eggs; repeat it now and then.
Butter in blocks
The block has its weight on the wrapper. Price per gram = block price ÷ grams in the block. Weigh one now and then to check it matches.
If your recipe says "half a block" or "a stick", change it to grams on your recipe card. Then if you buy a different size tomorrow, the recipe and the cost stay put. Leftover butter keeps in the fridge and goes into the next batch, so each recipe pays only for the grams it uses.
Milk and cream: the opened carton
Price per millilitre or per gram is simple: pack price ÷ contents. What changes the cost is what is left over. An opened carton of milk, a tub of cream cheese or a pot of cream does not last long. If the rest goes into something else, each recipe pays for its share. If it goes in the bin, the recipe that opened it pays for the whole pack.
Do not take volume-to-weight conversions from a chart. If your recipe says "one cup of milk", measure your cup on your scale once and write the grams on your card. The guide to units and conversions for costing explains how to build your own equivalents.
Cheese: what you pay for is not what you use
Cheese loses weight before it reaches the plate. Rind and ends get cut off, some stays on the grater, slices break, fresh cheeses release liquid. Measure your own case: weigh what you buy and weigh what you actually use.
cost per usable kilo = price per kilo ÷ (usable grams ÷ grams bought)
If you buy a 1 kg block of cheddar at $11.00 for your toasties, and after trimming and grating you weigh 940 g, your real cost is 11.00 ÷ 0.94 = $11.70 per usable kilo. The yield tool does that sum: real cost per usable kilo after trim and loss.
Worked example: a batch of cupcakes
You bake vanilla cupcakes with cream cheese frosting, 12 to a batch. Here is how you buy and use the eggs and dairy:
| Ingredient | How you buy it | Price per unit used | Recipe | Cost |
|---|---|---|---|---|
| Eggs | tray of 30 large at $9.00 | 9.00 ÷ 30 = $0.30 per egg | 3 eggs | $0.90 |
| Butter | 250 g block at $3.50 | 3.50 ÷ 250 = $0.014 per g | 170 g | $2.38 |
| Milk | 1 litre carton at $1.40 | 1.40 ÷ 1,000 = $0.0014 per ml | 120 ml | $0.17 |
| Cream cheese | 200 g tub at $2.80 | whole tub | 200 g | $2.80 |
| Total | $6.25 |
The sums: 3 × 0.30 = 0.90; 170 × 0.014 = 2.38; 120 × 0.0014 = 0.168; the tub is used whole, 2.80. Total 0.90 + 2.38 + 0.168 + 2.80 = $6.248, so $6.25 a batch and 6.25 ÷ 12 = $0.52 per cupcake in eggs and dairy alone.
If the rest of the milk goes off. You use 120 ml and the other 880 ml sours before you get to it. Then the batch pays for the whole carton: 0.90 + 2.38 + 1.40 + 2.80 = $7.48 a batch, $0.62 per cupcake. That is $1.23 more per batch for one wasted carton. The answer is planning: use the rest the same day in another bake, or buy a size you will finish.
If you switch to medium eggs. Three medium eggs at 8.10 ÷ 30 = $0.27 each cost $0.81, so the batch looks $0.09 cheaper. But there is less egg in it, and per kilo the medium tray was the more expensive one. Counting eggs hides that; weighing shows it.
With the recipe in grams. If you write the recipe as 165 g of egg without shell, and you measured 880 g usable per kilo of large eggs, the cost is 4.84 ÷ 0.88 = $5.50 per usable kilo, and 0.165 × 5.50 = $0.91. That figure does not move with egg size, only with the price per kilo.
Pack and recipe card
Each of these ingredients is bought in one pack and used in another measure. The guide to costing an ingredient you buy by the package has the general method; the only twists here are that eggs vary in size, opened milk does not keep and cheese has loss. To see dairy alongside meat, produce and the rest, see ingredient costs by category.
Do it with the calculator
In the food cost calculator, enter each ingredient with its purchase price and the recipe quantity in the same unit: eggs by count or in grams, butter and milk in grams or millilitres, cheese at its price per usable kilo. When the price of a tray changes, you update one number and the batch is recalculated.
Frequently asked questions
How much does an egg weigh for costing?
It depends on the eggs you buy that week, so weigh them yourself. Weigh a dozen, divide by 12 and write it down. Do it again when you change size or supplier.
Should a recipe list eggs by count or by weight?
For baking, by grams of egg without shell. The recipe comes out the same whatever the egg size, and so does the cost. For savoury cooking, counting eggs is usually enough.
How do I convert a cup of milk to grams?
Measure it with your own cup and scale: fill the cup the way you do when cooking, weigh it and subtract the empty cup. Write that number on your recipe card and use it every time.
Is pre-grated cheese cheaper than a block?
Compare the cost per usable kilo. The block loses some weight to rind, ends and grating, which you can measure; the bag is already usable. Divide each price by the grams you actually use.