A unit price is the price divided by the quantity: unit price = price ÷ size. It lets you compare packages of different sizes on equal terms. A 24 oz jar at $4.99 costs $0.2079 per ounce; a 40 oz jar at $7.49 costs $0.1873 per ounce, so the bigger jar is 9.9% cheaper per ounce.
NIST describes unit pricing as the price per unit of measure (for example per gram, liter or ounce) shown on shelf labels and online. NIST also points to unit prices as a way to spot "shrinkflation" (a smaller package at the same price). This calculator converts all sizes to a common unit before comparing. Enter metric and U.S. sizes freely, as long as all products are measured the same way (weight, volume or count).
How to use the unit price calculator
- Enter the price and size of product A and product B (and optionally C).
- Pick each size's unit; oz means weight ounces, fl oz means fluid ounces.
- Read which product is cheaper per unit and by what percentage, and the table of unit prices.
Formula
Weights are converted through grams (1 oz = 28.3495 g, 1 lb = 453.592 g) and volumes through milliliters (1 fl oz = 29.5735 mL, 1 gal = 3,785.41 mL).
Worked example
Peanut butter: A is 24 oz for $4.99; B is 40 oz for $7.49.
- A: $4.99 ÷ 24 = $0.2079/oz ($3.33/lb).
- B: $7.49 ÷ 40 = $0.1873/oz ($3.00/lb).
- B is 1 − 0.18725 ÷ 0.20792 = 9.9% cheaper per ounce.
When the bigger package isn't the better buy
Larger sizes are often cheaper per unit, but not always; sales and coupons on smaller packages can flip the result. And a lower unit price only saves money if you use the extra before it spoils. Add a coupon or sale to the price before comparing (the discount calculator can work out the sale price), and remember that sales tax applies the same way to both. For different measures of the same kind (say, grams vs. ounces), this calculator converts for you; to convert other amounts, see the weight converter and volume converter.
Multipacks and count items
For products sold by count (paper towels, diapers, batteries, eggs), use "count / items" and enter the total number of units. A 12-pack of 6 rolls is 72 rolls only if you are comparing the same roll size; if roll sizes differ, compare by sheet count or by area instead. For multipacks of cans or bottles, multiply the number of containers by the size of each one: 12 cans × 12 fl oz = 144 fl oz.
Mixed units example
Olive oil: A is 500 mL for $8.99; B is 25.5 fl oz for $12.49. Choose mL for A and fl oz for B. A works out to $1.80 per 100 mL ($0.53 per fl oz) and B to $0.49 per fl oz, so B is cheaper per unit by about 8% even though its sticker price is higher.
Caveats and assumptions
- Compare like with like: weight with weight, volume with volume, count with count.
- Check whether a shelf label uses net weight, drained weight or count.
Frequently asked questions
How do you calculate unit price?
Divide the price by the quantity. $4.99 for 24 oz = $0.208 per ounce.
How do I compare products in ounces and grams?
Convert both to the same unit first (1 oz = 28.35 g). This calculator does it automatically.
Is the bigger size always cheaper?
No. It often is, but sales and coupons can make a smaller size cheaper per unit, so compare the unit prices.
What is a good unit to compare by?
Use the one on the shelf label when possible, such as per ounce, per pound, per 100 g, per fluid ounce or per item.
Can I compare a metric size with an ounce size?
Yes. Pick the right unit for each product; the calculator converts both to a common unit before comparing.
Sources
- NIST — Uniform Unit Pricing: Tools for Consumers to Fight Shrinkflation — what unit pricing is and how labels show it
- NIST SP 1181 — Unit Pricing Guide (2025 ed.) — best-practice unit price labeling
- NIST SP 811 Appendix B.9 — Conversion factors — ounce, pound, fluid ounce and gallon factors
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.