Quick answer: Divide each price by its size in the same unit, then compare. 5 lb for $4 is $0.80 per lb and 4 lb for $2.40 is $0.60 per lb, so the smaller bag is cheaper. Bigger is usually cheaper per unit, but not always, and only if you use it all.
Short answer: divide each price by its size, in the same unit, and pick the lowest result. For 5 lbs for $4 versus 4 lbs for $2.40, the math is $4 ÷ 5 = $0.80 per lb and $2.40 ÷ 4 = $0.60 per lb. The 4 lb bag is cheaper by 20 cents per pound, which is 25% less. The unit price calculator does the division for you and handles mixed units.
The idea is simple, but real shelves are messy: bags in pounds, boxes in ounces, bottles in liters, and packs by count. This guide shows a repeatable method and 12 worked examples you can copy.
The 4-step method
- Pick one unit. Choose ounces, pounds, fluid ounces, or count. Convert everything to it.
- Find the total quantity. For multipacks, multiply the number of packages by the size of each.
- Divide price by quantity. The result is the unit price.
- Compare, then adjust. Choose the lowest unit price, then ask whether you will actually use all of it.
The formula, used by university extension programs such as NDSU, is simply: item price ÷ item size = unit price. For example, $2.99 for a 12 oz package is $2.99 ÷ 12 = $0.249 per ounce.
Handy conversions
| To compare in… | Use this |
|---|---|
| Pounds | 16 oz = 1 lb (multiply price per oz by 16) |
| Fluid ounces | 32 fl oz = 1 quart, 128 fl oz = 1 gallon |
| Liters | 1 L is about 33.8 fl oz |
| Dozens | 1 dozen = 12 |
| Grams | 28.35 g = 1 oz, 454 g = 1 lb |
For a full chart, see ounces to cups and other cooking conversions or use the grocery & cooking unit converter.
12 worked examples
1. 5 lbs for $4 vs 4 lbs for $2.40 (same unit, different sizes)
- 5 lb at $4.00: $4.00 ÷ 5 = $0.80 per lb
- 4 lb at $2.40: $2.40 ÷ 4 = $0.60 per lb
Winner: 4 lb bag. It is 25% cheaper per pound. Buying 4 lb costs $2.40, while 4 lb from the bigger bag’s price rate would cost $3.20. Bigger is not always cheaper.
2. The ketchup example from NIST
The National Institute of Standards and Technology (NIST) uses this example: a 32 oz bottle at $4.99 costs about $0.16 per ounce, while a 14 oz bottle at $3.19 costs about $0.23 per ounce.
Winner: 32 oz bottle, about 32% cheaper per ounce. NIST notes that for the most part, the larger size is the better value, and unit price labels make it easy to confirm.
3. Ounces vs pounds (different units)
- Box, 18 oz at $3.79: $3.79 ÷ 18 = $0.211 per oz
- Bag, 2 lb at $5.49: 2 lb = 32 oz, so $5.49 ÷ 32 = $0.172 per oz
Winner: 2 lb bag, about 19% cheaper per ounce. Converting to the same unit is the whole trick.
4. Fluid ounces vs quarts
- 12 fl oz at $2.49: $2.49 ÷ 12 = $0.208 per fl oz
- 1 quart (32 fl oz) at $5.99: $5.99 ÷ 32 = $0.187 per fl oz
Winner: quart, about 10% cheaper.
5. Eggs: a dozen vs 18
- 12 eggs at $3.49: $3.49 ÷ 12 = $0.291 each
- 18 eggs at $4.89: $4.89 ÷ 18 = $0.272 each
Winner: 18-count, about 7% cheaper per egg. Eggs last weeks, so the bigger carton rarely goes to waste.
6. Multipack cans vs a 2-liter bottle
- 12-pack, 12 fl oz cans at $7.99: total 144 fl oz, so $0.055 per fl oz
- 2-liter bottle at $2.29: 2 L is about 67.6 fl oz, so $0.034 per fl oz
Winner: the 2-liter bottle, about 39% cheaper. The pack’s convenience costs real money.
7. Paper towels: price per sheet
- 6 rolls × 100 sheets at $8.99: 600 sheets, so 1.50 cents per sheet
- 8 rolls × 70 sheets at $7.49: 560 sheets, so 1.34 cents per sheet
Winner: 8 rolls, about 11% cheaper per sheet. Roll counts mislead when the sheets per roll differ. Compare the total sheets.
8. Produce: per pound vs bagged
- Loose apples at $1.99 per lb
- 3 lb bag at $4.99: $4.99 ÷ 3 = $1.66 per lb
Winner: the bag, about 16% cheaper per pound, as long as you will eat all 3 pounds.
9. The bulk size you cannot finish
- 10 lb bag of potatoes at $5.99: $0.60 per lb
- 5 lb bag at $3.49: $0.70 per lb
On paper, the 10 lb bag wins. If 3 pounds spoil, you used 7 pounds for $5.99, which is $0.86 per usable lb, worse than the 5 lb bag if you eat all of it. Unit price only counts if the food gets eaten. See is buying in bulk actually cheaper for the full waste math.
10. Name brand with a coupon vs a store brand
- Name-brand cereal, 15 oz at $3.99 with a $1 coupon: $2.99 ÷ 15 = $0.199 per oz
- Store-brand cereal, 18 oz at $3.29: $3.29 ÷ 18 = $0.183 per oz
Winner: store brand, about 8% cheaper even after the coupon. Read more in store brand vs name brand.
11. A sale on the small size
- Regular 24 oz at $4.99: $0.208 per oz
- Sale 16 oz at $2.99: $0.187 per oz
Winner: the sale item, about 10% cheaper per ounce. A sale on the smaller size can beat the larger size at regular price.
12. Shrinkflation: same price, less product
A product that used to be 16 oz at $4.00 ($0.25 per oz) is now 14 oz at $4.00 ($0.286 per oz). The price did not change, but the unit price rose 14%. NIST describes this practice, known as shrinkflation, as reducing package contents without changing the price. See what is shrinkflation and how to spot it.
When the bigger size is not the better deal
- Odd pack sizes. A pack of 10 might cost more per unit than a pack of 8.
- Sales on small packs or loyalty-card prices.
- Store brands that beat name brands in any size.
- Short shelf life. Fresh produce, bread, and dairy can spoil before you finish a large package. Check the food shelf life calculator.
- Storage limits. If you have no room, you will not buy it again at the low price.
- Different quality or concentration. Laundry detergent, juice concentrate, and spices need a per-use or per-serving comparison, not per ounce.
Are shelf-tag unit prices reliable?
Mostly, but not always. Unit pricing is not federally mandated. NIST reports that unit pricing rules are decided state by state: nine states and one territory require it, eight states and two territories have voluntary rules, and thirty-three states and three territories have none, though many stores display it anyway. Tags can show different units on neighboring products, and some are outdated after a price change. When something looks odd, do the quick math yourself.
Compare per serving, not only per ounce
Per-ounce math works when products are the same. For different foods, think per serving or per use:
- Protein: price per serving of 20 grams of protein, rather than price per package.
- Cleaning products: price per load or per use.
- Snacks: price per portion actually eaten.
The recipe cost calculator and price comparison calculator help here. If you compare often, a price book lets you record unit prices once and spot a real sale at a glance.
Common mistakes
- Comparing weight to volume. Ounces (weight) and fluid ounces (volume) are not the same, so do not compare a bag of rice to a bottle of oil.
- Ignoring pack size. Multiply units by size before dividing.
- Trusting the sticker size. Check the net weight, which can change between versions of the same product.
- Forgetting the waste. The cheapest unit price is the one you actually use.
- Assuming loyalty or sale prices stack. Confirm what price you will pay at the register.
Quick cheat sheet
- Unit price = price ÷ quantity
- Price per pound = price per ounce × 16
- Multipack quantity = number of packs × size of each
- Percent saved = (higher unit price − lower unit price) ÷ higher unit price
- Always check: same unit, same product type, and will you use it all?
Limits of these examples
The prices in the examples are illustrative, chosen to show the method, and are not current store prices. Use the numbers on your own store’s shelf tags.
Sources
- National Institute of Standards and Technology, Best Practices for Uniform Unit Pricing: An Update to NIST SP 1181 and NIST Handbook 130 Regulations
- NIST, Handbook 130, Uniform Unit Pricing Regulation
- North Dakota State University Extension, 6 Tips to Save Money With Unit Pricing
Last reviewed October 7, 2026.
Evergreen method
Compare usable cost, not package price
The durable comparison is price per common unit adjusted for the share you will actually use.
Formula
usable unit cost = price ÷ (quantity × usable share)Reproducible steps
- 1.Convert every option to the same unit.
- 2.Estimate the share your household will use before spoilage.
- 3.Choose the lower usable unit cost only if storage and cash flow also work.
Put it to work
Run the numbers for your household
FAQ
Common questions
Which is cheaper, 5 lbs for $4 or 4 lbs for $2.40?
4 lbs for $2.40 is cheaper. Divide each price by its weight: $4 ÷ 5 lb is $0.80 per pound, and $2.40 ÷ 4 lb is $0.60 per pound. The 4 lb bag costs 25% less per pound, even though it is the smaller bag.
How do you compare prices of different sizes?
Divide the price by the quantity to get a unit price, such as price per ounce or per pound, for each option. Make sure both use the same unit, then pick the lowest number. This works for any size, brand, or package.
Is the bigger package always cheaper per unit?
No. Larger sizes are usually cheaper per ounce, but not always. Sales, coupons, store brands, and odd pack sizes can make a smaller package the better value. Check the unit price on the shelf tag or calculate it yourself instead of assuming.
How do I compare different units like ounces and pounds?
Convert both to the same unit first. There are 16 ounces in a pound, so multiply a price per ounce by 16 to get price per pound. For liquids, use fluid ounces: 32 in a quart, 128 in a gallon, and about 33.8 in a liter.
How do I calculate price per ounce?
Divide the price by the number of ounces. A 24 oz jar for $4.99 is $4.99 ÷ 24 = $0.208 per ounce, or about 21 cents. For a multipack, multiply the number of units by the size of each unit to get the total ounces first.
How do I compare multipacks to single items?
Work out the total quantity in the pack and divide the price by it. A 12-pack of 12 fl oz cans is 144 fl oz in total, so a $7.99 pack is about $0.055 per fluid ounce. Compare that to a 2-liter bottle at $2.29, which is about $0.034 per fluid ounce.
Are unit prices on shelf tags required by law?
Not federally. According to the National Institute of Standards and Technology, unit pricing rules are set by each state: nine states and one territory require it, eight states and two territories have voluntary rules, and the rest have none. Many stores show it anyway.
Is it worth buying a bigger size with a lower unit price?
Only if you will use it all before it spoils or loses quality. A 10 lb bag of potatoes at $0.60 per pound is not cheaper than a 5 lb bag at $0.70 per pound if you throw away 3 pounds, because you paid for 10 lb but used 7.
Does a sale price always beat the regular unit price?
No. A sale is only a good deal if the unit price after the discount is the lowest option. Compare the sale item's price per unit to the regular price per unit of other sizes and store brands before you buy.
What if the shelf tag unit price uses different units?
Convert them yourself. One tag might show price per ounce and another price per pound, or one per 100 sheets and another per roll. Pick one unit, convert the other, and compare. A unit price calculator does this automatically.
Related
Keep reading
Unit Price Explained: How to Always Spot the Cheaper Option
What unit price actually means, why sticker price is misleading, and how to compare two products in seconds without doing math in the aisle.
Is Buying in Bulk Actually Cheaper?
When bulk buying genuinely saves money and when it quietly costs more once spoilage and unused product are factored in.
What Is Shrinkflation? Examples, How to Spot It, and How to Protect Your Budget
Shrinkflation means less product for the same price. See how to calculate the real price hike (16 oz to 14 oz is +14%), which items shrink most, and how to spot it.
Ounces to Cups and Other Cooking Conversions: Charts and Cheat Sheet
How many ounces in a cup? 8 fluid ounces, but a cup of flour weighs about 4 1/4 oz. Charts for cups, tablespoons, pounds, grams, and ingredient weights.