The one formula that matters
Unit price = price ÷ size, where size is measured in one consistent unit. That's the entire method. A $4.99 box of 12 oz cereal is $4.99 ÷ 12 = $0.42 per ounce. A $7.99 box of 20 oz is $7.99 ÷ 20 = $0.40 per ounce. A $11.49 box of 32 oz is $11.49 ÷ 32 = $0.36 per ounce. The biggest box wins by six cents an ounce over the smallest — small per box, meaningful over a year of groceries.
Read the shelf tag first
Many stores print the unit price on the shelf tag — the small number under the big price. Start there; it saves the mental math. But read the unit the tag uses, because tags sometimes switch between per-oz, per-lb, and per-100-count precisely to make comparisons awkward. If one tag says "$0.42/oz" and another says "$5.99/lb," you're being shown two different denominators on purpose. Convert one: $0.42/oz × 16 = $6.72/lb — suddenly the $5.99/lb tag is the clear winner. Never compare two tags that use different units without converting.
Worked example: three boxes of cereal
Box B: $7.99 ÷ 20 oz = $0.400/oz
Box C: $11.49 ÷ 32 oz = $0.359/oz
Savings, C vs A: $0.416 − $0.359 = $0.057/oz — about $1.82 saved per 32 oz box
Answer: Box C wins, and the middle box — the one that looks like the sensible compromise — is barely cheaper per ounce than the small box. Run any three sizes through the unit price calculator to get the ranked answer instantly, with mixed units handled for you.
The middle-size trap
Shoppers assume price-per-unit falls smoothly as size grows. Retailers know this, so the middle size frequently carries the highest margin per unit of the lineup: the small size is priced to look affordable, the big size to look like a deal, and the middle one coasts on the assumption that it splits the difference. Always calculate all three — the "compromise" size is the one most likely to be quietly overpriced.
Sales and coupons flip the math
Unit price is a snapshot, not a permanent property of a product. A 20%-off small pack routinely beats the full-price bulk pack, and a coupon on the middle size can dethrone the big one. The rule: apply the sale price first, then divide. If the sticker says 25% off $7.99, the real price is $5.99 — run $5.99 ÷ 20 oz and compare that. Never assume the bulk pack stays the winner once red stickers appear.
Shrinkflation: the quiet price hike
Shrinkflation is a price increase wearing a "new look" costume: the package shrinks from 12 oz to 10 oz while the price stays $4.99. The unit price jumps from $0.42 to $0.50 per ounce — a 20% hidden increase. Catch it with two numbers: the size change (10 − 12) ÷ 12 = −16.7%, and the unit-price change ($0.50 − $0.42) ÷ $0.42 = +20%. When the pack shrinks and the unit price rises, that's shrinkflation, full stop. The calculator's shrinkflation tab does both divisions for you.
When the cheapest per unit loses
The waste rule overrides everything: only buy the cheapest per unit among sizes you'll finish. A bulk bag of spinach that wilts, a gallon of milk that sours, a warehouse pack of berries that molds — food you throw away is infinitely expensive per unit eaten. Perishables, items you buy to try for the first time, and anything with a short shelf life should be sized to your actual consumption, not to the unit-price ranking. Frozen produce is the classic exception: often 30–50% cheaper per pound than fresh, with zero spoilage waste since you use only what you need.
Try it: line up three sizes in the free unit price calculator — it converts ounces, pounds, grams, and milliliters for you and ranks the options with a live leaderboard — or run the shrinkflation check on any pack that got suspiciously smaller.