Divide the price by a comparable quantity
To compare sale prices using unit price, calculate the discounted pack price first, divide it by the quantity in the pack, and express every option on the same basis. Price per kilogram, price per liter and price per item are common choices. The lower total pack price is not necessarily the lower unit price because a cheaper package can contain less. Comparing equivalent quantities makes that difference visible.
Use this method for sufficiently comparable products with known quantities and one currency. A unit price supports a price comparison; it does not establish equal quality, concentration or suitability. Two bottles labelled with the same volume may perform differently if their recommended dilution differs. Identify the quantity that answers your question before dividing. The calculation can be exact for the supplied numbers while the choice of comparison basis still needs judgment.
Choose one measurement basis before calculating
If one bag contains 750 g and another 1.2 kg, convert both to grams or kilograms. For a price per kilogram, divide price by kilograms; for a price per 100 g, divide price by grams and multiply by 100. These expressions represent the same relative price when converted consistently. A price per 100 g must not be compared numerically with a price per kilogram without the factor of ten.
Mass and volume are different quantities. A 500 g package cannot be compared directly with a 500 mL package as though their contents were identical; a defensible conversion would require relevant density or product information. Ounces can also mean mass ounces or fluid ounces. Fluid ounces may refer to different systems. Keep the actual measurement label from the package and use a suitable unit converter where a genuine like-dimension conversion is needed.
Compare two discounted package sizes
Suppose option A is a 750 g package originally priced at 6.40 with 25% off. Its sale price is 6.40 × 0.75 = 4.80. Divide by 0.75 kg to obtain 6.40 per kilogram. Option B contains 1.2 kg, originally costs 8.00 and has 10% off. Its sale price is 7.20, or 7.20 / 1.2 = 6.00 per kilogram. Option B has the lower unit price despite its smaller discount percentage.
The percentage difference in unit prices uses a specified reference. Compared with A, B is (6.40 − 6.00) / 6.40 = 6.25% cheaper per kilogram. That does not mean its pack costs less: B requires 7.20 at checkout and A requires 4.80 before extras. Keep the purchase quantity and total spend beside the normalized rate so a useful comparison does not disguise an amount beyond the intended budget.
| Option and promotion | Sale price / quantity | Comparable unit price |
|---|---|---|
| A: 6.40 less 25% | 4.80 / 0.75 kg | 6.40 per kg |
| B: 8.00 less 10% | 7.20 / 1.2 kg | 6.00 per kg |
B is 6.25% cheaper per kilogram relative to A, while A requires less money for one pack. Excludes tax and fees.
Expand multipacks into total contained quantity
A multipack containing four 250 mL bottles has total volume 1,000 mL, or 1 L. If its sale price is 3.60, the unit price is 3.60 per liter, not 14.40 per liter. A separate 1.5 L bottle priced at 4.95 is 3.30 per liter. The larger bottle is cheaper per liter under these numbers, while the multipack may serve a different practical need. Quantity normalization identifies the price difference without deciding that preference.
For counted goods, use a consistent count such as price per tablet or per sheet, checking that the units themselves are comparable. A pack's number of containers is not always the relevant denominator. Four bottles of different sizes cannot be treated as four equal portions by count alone. If a bonus quantity is included, add it to the quantity actually received only when it is the same usable product on the chosen measurement basis.
Apply the discount before dividing by quantity
For a single percentage reduction on the whole package, unit price equals original price × (1 − discount fraction) / quantity. You can also discount a correctly calculated original unit price by that fraction, obtaining the same result before rounding. A fixed coupon must be allocated to the eligible purchase amount first. Its value divided by an unrelated package quantity does not determine the final unit price of every item in a mixed basket.
The Discount Calculator performs the price step, not the quantity-normalization step. For option A, enter original 6.40 and percentage 25, then record the final 4.80. Divide that amount by 0.75 kg separately. The tool has no package-size or unit-price field. If offers stack, use the supported successive-percentage mode for eligible percentage reductions, then normalize the final price; do not add percentage reductions or assume a coupon applies to all products.
Distinguish label quantity from useful quantity
Suppose you buy option B for 7.20 but use only 0.9 kg before the rest is discarded. Its purchase cost per useful kilogram is 7.20 / 0.9 = 8.00, higher than its label-based 6.00 per kilogram. This is a scenario about usage, not a correction to the retailer's quantity label. Keep both numbers if you need to understand the effect of waste rather than silently replacing the advertised package weight.
Concentrated products can require another defensible denominator, such as stated doses, but follow the relevant product instructions and preserve their assumptions. A calculated price per suggested use should not be presented as a universal performance equivalence across brands. For food with inedible parts or drained contents, state which quantity is being compared. If useful quantity is unknown, show a reasonable scenario range instead of claiming a precise waste-adjusted rate.
Separate item unit price from acquisition cost
Delivery or a compulsory fee can change the total cost of obtaining a package. For a single-item order, adding a 2.00 delivery charge to option A produces 6.80 for 0.75 kg, approximately 9.0667 per kilogram acquired. That is an acquisition-cost scenario rather than the original item unit price. For an order containing multiple goods, any allocation of a shared charge should be stated, since several reasonable allocations may answer different questions.
Compare taxes and other charges on the same basis across alternatives without assuming one universal checkout rule. Currency changes also require an explicit exchange-rate source and time if you compare countries; the Discount Calculator's currency selector only changes labels. This guide does not determine tax law, verify advertised reference prices or recommend a product. It provides arithmetic for a clearly defined comparison once price, quantity and eligibility are known.
Keep enough precision to rank close offers fairly
Retain unrounded unit prices until deciding the comparison, then display a sensible number of decimals. Two options both shown as 0.03 per gram may have slightly different rates hidden by that display. Converting to a larger base, such as price per kilogram, can make the difference readable. Avoid rounding a per-gram value first and multiplying the rounded number into a purported exact per-kilogram price.
A useful comparison record includes product identity, pack quantity, normal and sale prices, discount conditions, standardized unit and any separate usage or acquisition assumptions. For the 750 g and 1.2 kg examples, write 6.40/kg versus 6.00/kg alongside 4.80 versus 7.20 paid. That paired presentation explains both affordability and normalized price. Recheck the shelf or checkout amount before relying on an outdated offer, especially when eligibility or package sizes have changed.