Numvolt · Guides
Unit price calculator: compare prices per kg or litre
Compare two pack sizes on the same unit basis, follow the division and check quantities, rounding and limits.
In this guide6
Compare unit prices
Enter two prices in the same currency and quantities for the same kind of product. This calculator compares the price per kilogram or litre.
The calculator needs JavaScript. You can also divide each price by the quantity in kilograms or litres using the examples below.
Use a comma or a decimal point, without thousands separators. Prices must use the same currency.
Calculations run in your browser. Your entries are neither saved nor sent. These example prices are fictional. Consider product quality and the quantity you need as well as unit price.
Compare the quantity you actually get
A lower shelf price can belong to a smaller pack. Unit pricing compares the cost of the same amount: a kilogram, litre or another consistent unit. The NIST guide and the UK Competition and Markets Authority explain this common basis. Start with two comparable products in the same currency, and use the price and net quantity you would actually pay for. These examples are fictional; they do not report current shop prices.
Sources for this section NIST Unit Pricing Guide, 2025 · UK CMA: Unit pricing information for consumers
Convert the quantity before dividing
For a price per kilogram, divide grams by 1,000. A 450 g pack is 0.45 kg. For a price per litre, divide millilitres by 1,000: 250 mL is 0.25 L. Keep the unit beside each number. Compare mass with mass and volume with volume. A gram is not automatically a millilitre; converting between those requires additional information about the substance.
Sources for this section NIST Unit Pricing Guide, 2025
Check equivalent fractions and decimalsWork through two different pack sizes
Suppose pack A costs 3.60 for 450 g and pack B costs 4.00 for 500 g. Calculate A: 3.60 ÷ 0.45 = 8.00 per kg. Calculate B: 4.00 ÷ 0.50 = 8.00 per kg. They have the same price per kilogram even though B costs more at the checkout. Check by multiplying back: 8 × 0.45 = 3.60 and 8 × 0.50 = 4.00.
| Pack | Price | Quantity in kg | Price per kg |
|---|---|---|---|
| A | 3.60 | 0.45 | 8.00 |
| B | 4.00 | 0.50 | 8.00 |
Sources for this section NIST Unit Pricing Guide, 2025
How to divide numbers in your head, with worked examplesUse a second example and keep rounding separate
A 250 mL bottle at 2.50 costs 2.50 ÷ 0.25 = 10.00 per L. A 400 mL bottle at 3.60 costs 3.60 ÷ 0.40 = 9.00 per L. The second has the lower unit price for these inputs. Calculate before rounding the display: similar-looking rounded prices need not be equal. If a calculator reports a tie, inspect its precision and the entered quantities before deciding the prices are identical.
Sources for this section UK CMA: Unit pricing information for consumers
Check what the comparison leaves out
Unit price compares quantity and price, not quality or suitability. A larger pack may be more than you can use. Enter a promotion only when you know the quantity and total price covered by its conditions. Delivery charges, deposits, excluded items and different currencies can change the question. NIST recommends consistent unit presentation; this worksheet is arithmetic, not a statement of every country’s labelling law.
Sources for this section NIST Unit Pricing Guide, 2025 · UK CMA: Unit pricing information for consumers
Turn the calculation into practice
Try the division first, check it with a calculator, then explain a mismatch: wrong unit, wrong quantity or arithmetic. Numvolt offers practice with the four operations, an untimed Zen mode and explanations after mistakes. This independent worksheet does not mean Numvolt includes a shopping or unit-price module. Practise the underlying division there, and use the real label or this page when comparing a specific pair of packs.
Sources for this section Numvolt product features, publisher source
Questions
How do I calculate the price per kilogram?
Convert grams to kilograms, then divide the price by that quantity. For 3.60 and 450 g: 3.60 ÷ 0.45 = 8.00 per kg.
Is a bigger pack always cheaper per unit?
No. Compare both divisions on the same unit basis; size alone does not decide the result.
Can I compare grams with millilitres?
Not directly. They measure different quantities; a mass-to-volume conversion needs information about the substance.
How do I change a price per 100 g to a price per kg?
Multiply it by 10, because a kilogram contains ten lots of 100 g. Keep the currency unchanged.
What if the two displayed prices look equal?
Check unrounded values and input precision. A rounded display can hide a small difference.
Does Numvolt include this shopping calculator?
This page is an independent worksheet. Numvolt practises arithmetic; do not assume a shopping module from this guide.
Further reading
Guides
How mental arithmetic works in your head, and why some methods are easier
Retrieval versus computation, where working memory runs out, why carrying is the expensive part, why anxiety costs accuracy, and what all of that means for how you practise.
Negative numbers: check the sign before calculating
Distinguish a negative sign from subtraction, check worked integer examples and enter a signed result in Numvolt.
Mental math practice without guessing: a better daily loop
Use mental arithmetic to practise methods, check errors and build speed without confusing a fast guess with a reliable answer.