What this calculates
Enter the price and size of two packs — each in its own unit, such as 500 g against 1 kg — and this works out the price per kilogram, per litre or per piece for each, tells you which is the better buy, and by how much per unit.
The formula
Unit price
price ÷ size in kilograms, litres or piecesHow much cheaper
(dearer unit price − cheaper unit price) ÷ dearer unit price × 100
Grams are divided by 1,000 to give kilograms, and millilitres by 1,000 to give litres, so packs of different sizes can be set side by side.
Worked example
The 1 kg pack is the better buy, at ₹20 less per kilogram. For eggs, 12 for ₹72 is ₹6.00 each and 6 for ₹35 is ₹5.83 each, so the smaller box is 2.78% cheaper per egg.
When to use it, and the mistakes to avoid
Use it in the supermarket or when shopping online to compare pack sizes, brands or offers — rice, oil, detergent, snacks, anything sold by weight, volume or count.
The mistakes that cost the most:
- Comparing prices, not unit prices. ₹220 looks dearer than ₹120 until you see it buys twice as much.
- Assuming the big pack wins. Small packs on offer are sometimes cheaper per kilogram.
- Mixing units. 500 g is 0.5 kg, not 500 kg; pick the right unit for each pack.
- Comparing weight with volume. Grams and millilitres cannot be compared without the product's density.
- Buying more than you will use. Food thrown away wipes out any saving.
FAQ
How do I work out the unit price?
Divide the price by the size in a common unit. 500 g for ₹120 is 0.5 kg, so ₹120 ÷ 0.5 = ₹240 a kilogram. 1 kg for ₹220 is ₹220 a kilogram, so the 1 kg pack is cheaper.
Is the bigger pack always cheaper?
Often, but not always - offers on small packs and 'family packs' priced higher per kilogram are both common. Comparing the price per kilogram, litre or piece is the only way to know.
How much cheaper is one pack than another?
Take the difference in unit price and divide by the dearer one. At ₹240 against ₹220 a kilogram, the difference is ₹20, and ₹20 ÷ ₹240 = 8.33% cheaper.
Can I compare grams with millilitres?
Not directly. Grams measure weight and millilitres measure volume, and how they relate depends on the product - a litre of oil weighs less than a kilogram. Compare two packs measured the same way.
Does the shop label show the unit price?
In India, yes: since October 2022 the Legal Metrology (Packaged Commodities) Rules require a unit sale price on every pre-packed product, beside the MRP. But it is per gram for packs under 1 kg and per kilogram above, so a 500 g pack reads ₹0.24 per g and a 1 kg pack ₹220 per kg - not easy to compare at a glance. This calculator puts both on the same unit.
When is the cheaper unit price not the better buy?
When you will not use it all. A bigger pack that spoils or expires before you finish it costs more per unit you actually use. It is also worth checking quality is the same.