Milk fat standardisation: calculations with worked examples

Guide · Published 1 October 2026 · Worked examples are calculated with the pearson square calculator

Standardising milk means adjusting its fat to a target, such as 3.5% or 3.25%, by adding skim milk, adding cream, or taking cream off. Every case is the same mass balance: the fat going in equals the fat coming out. The Pearson square is the quickest way to do the arithmetic. Here are the four common cases, worked through.

The principle

kg of fat = kg of product × fat % ÷ 100 fat in = fat out and mass in = mass out

Fat percentages in milk are by mass, so the arithmetic is exact in kilograms. If you measure in litres, convert each stream with its own density first: milk is roughly 3% heavier than water, and cream is lighter than milk, so equal litres are not equal kilograms.

1. Lowering fat by adding skim milk

You have 1,000 kg of whole milk at 4.2% fat and want 3.5%, using skim at 0.05%.

Whole milk against skim: 3.45 parts whole milk to 0.7 parts skim.

Parts of whole milk = 3.5 − 0.05 = 3.45; parts of skim = 4.2 − 3.5 = 0.70.
The whole milk is fixed at 1,000 kg, so skim = 1,000 × 0.70 ÷ 3.45 = 202.9 kg.
Check: fat in = 1,000 × 4.2% + 202.9 kg × 0.05% = 42.10 kg; batch = 1202.9 kg; 42.10 kg ÷ 1202.9 kg = 3.50%.

2. Raising fat by adding cream

1,000 kg of milk at 3.4% fat, target 4.0%, with cream at 40%:

Parts of cream = 4.0 − 3.4 = 0.6; parts of milk = 40 − 4.0 = 36.0.
Cream = 1,000 × 0.6 ÷ 36.0 = 16.67 kg, giving 1016.67 kg at 4.0%.

3. Lowering fat by taking cream off

In a plant, high-fat milk is often standardised by separating off cream rather than adding skim. Take 1,000 kg of 4.2% milk and remove x kg of 40% cream so that what remains is 3.5%:

1,000 × 4.2 = 40 × x + 3.5 × (1,000 − x) x = 1,000 × (4.2 − 3.5) ÷ (40 − 3.5) = 19.18 kg of cream

That leaves 980.82 kg of 3.5% milk and 19.18 kg of cream to sell or use elsewhere. Compare case 1: adding skim gives more standardised milk (1202.9 kg), removing cream gives less but yields cream.

4. Building milk from cream and skim

To make 100 kg of 3.25% milk from 38% cream and 0.05% skim:

Parts of cream = 3.25 − 0.05 = 3.20; parts of skim = 38 − 3.25 = 34.75.
Cream = 8.43 kg, skim = 91.57 kg.

Enter the two streams and the target fat; the calculator gives the parts, amounts for your batch and the full working. The "Milk fat" preset loads case 4.

Open the Pearson square calculator

Fat is not the only thing that changes

Adding cream or skim also changes protein and solids-not-fat (SNF). Standardising fat and protein, or fat and SNF, together is a two-target problem: it needs simultaneous equations or a third stream, not one square.
Use fat results from the same test method for every stream, and test the finished batch.
Legal minimum fat and labelling rules depend on the product and the country; check yours.

The general method, with feed examples and three-ingredient workarounds, is in how to use the Pearson square method.