Unit price explained: compare packs, bulk deals and discounts
Find the better-value pack using one measurement basis, then account for discounts, delivery and waste.
Compare equal units
Unit price = total price ÷ usable quantity. Convert both packs to the same unit first. For groceries, compare price per kilogram or litre; for identical items, compare price per item. Comparing a price per 100 g with a price per kg without converting creates a tenfold error. Mass and volume are not interchangeable without a valid density.
A supermarket example
Pack A costs 6 for 750 g. Its price per kilogram is 6 ÷ 0.75 = 8. Pack B costs 9 for 1.2 kg, or 7.50 per kilogram. B is 6.25% cheaper per kilogram relative to A, even though you pay more at checkout. Use the unit price calculator with matching units, and percentage change to express the difference against a clearly chosen baseline.
Apply discounts to the price you actually pay
A 20% discount followed by another 10% gives 0.80 × 0.90 = 0.72 of the original price, a 28% reduction, not 30%. Check the discount calculator for one reduction and apply sequential discounts in order. For multi-buy offers, divide the total amount paid by the total quantity received. Include mandatory delivery or fees consistently across alternatives.
Bulk is not always better
If 20% of a 1.2 kg pack will be wasted, only 0.96 kg is useful. Paying 9 gives an effective price of 9.375 per usable kilogram, more than pack A if A is fully used. Also compare quality, concentration, shelf life and storage needs. For detergent, cost per supported wash may be more useful than cost per litre.
A quick decision process
Record the final payable price, convert the quantity, calculate the unit price and check how much you will actually use. Do not round the converted quantity too early. Keep the same currency and tax basis. Put the expected spending into your monthly budget, and check business-day delivery estimates before choosing a supplier.
Common questions
Is the lower unit price always the better purchase? No. A larger pack may exceed your cash budget or expire before use.
Can I compare concentrates directly by volume? Only if they have comparable concentration and usage. Otherwise compare the recommended dose or usable output.
Reference
NIST unit-pricing guidance supports consistent, understandable measurement bases for price comparisons.