One division per package, and a habit that survives the store.
By David Liu, founder of DIY9lives. Last updated November 2, 2026.
A unit price is a price divided by a quantity. That is the entire method, and it is the reason a shelf label with a small number in the corner is worth more than any coupon. This guide covers the division, the conversions that make comparisons fair, and the cases where the cheapest unit price is still the wrong purchase. The unit price comparator does the arithmetic on several packages at once.
Take the price on the tag and divide by the quantity in the package. A 24 ounce jar at $4.99 is 20.8 cents an ounce. A 16 ounce jar at $3.49 is 21.8 cents. The bigger jar wins, and the margin is about a cent an ounce.
A cent an ounce sounds trivial until it is multiplied by consumption. If the household uses 48 ounces a month, the cheap option costs $9.98 and the expensive one $10.48. Across a year that is $6 on a single product — not a windfall, but the same exercise applied to a dozen staples is a grocery bill difference you can feel.
Comparisons only work when both sides are measured the same way. Three conversions cover most of a supermarket:
| From | To | Multiply by |
|---|---|---|
| Pounds | Ounces | 16 |
| Kilograms | Grams | 1,000 |
| Litres | Millilitres | 1,000 |
Price per 100 grams is the most useful single measure for dry goods, because it sits in a range where the numbers are easy to compare mentally: $0.45 per 100 g against $0.52 is a visible gap, whereas $0.0045 per gram is not.
Fluid ounces and weight ounces are not the same thing, which is the classic error with liquids. A fluid ounce is a volume; an ounce is a weight. For water they happen to be close, and for honey they are not.
"3 for $10" is only a price of $10 if you buy three. Some stores charge full price for a single unit; others apply the discount to one. The honest comparison is what you will actually pay, so enter the price you will be charged and the quantity you will carry out.
Shelf labels help when they exist and are accurate. Many stores print a unit price on the label, and most of the time it is right. It is worth spot-checking one, because a mislabelled unit price is exactly the sort of error that persists for months, and because the label is often computed on a different base than the one you want — per ounce when you cook by the cup.
Shrinkflation hides in plain sight here. A box that drops from 16 ounces to 14 at the same price is a 14% increase in unit price with no change on the tag you read first. Comparing unit prices over months is the only way to see it.
A 48 ounce jar at 23.9 cents an ounce is worse than a 24 ounce jar at 20.8 cents if the second half goes stale. Waste is a cost, and it is not on the label. Buy the size you will use within its shelf life, not the size with the best division.
Unit price compares quantity, not what is in the package. Two tins at the same price per ounce can differ in water content, grade, or how much of the tin is syrup. For anything where the drained weight matters, compare the drained weight instead — and that is printed on the label too.
Cost per use beats cost per ounce for anything consumed in units: detergent by the load, coffee by the cup, bin bags by the count. A concentrated detergent at a higher price per litre can still be cheaper per wash, which is the number you actually pay.
Most of the value comes from a rough comparison rather than an exact one. Two tricks cover the aisle:
The habit is what matters. Once you have divided a few times, the pattern becomes visible: the middle size is usually the best value, the largest size is usually priced for convenience, and the smallest size exists to make the middle one look reasonable. That is a merchandising decision, and the division is how you see through it.
Unit price is a decision tool at the shelf, and it becomes a diagnostic tool at home. Keep one week of receipts and check the items you buy every week. The exercise is not about finding waste; it is about noticing which products have the widest spread between the cheapest and dearest package size. Those are the ones where the choice is worth making deliberately rather than by habit.
Two patterns show up almost every time. Store brands frequently beat national brands on unit price by a margin that is larger than any coupon, and the gap is widest on staples — flour, oats, canned tomatoes, frozen vegetables. And the largest package is often the worst value per unit, because size signals convenience and the pricing follows the signal rather than the cost.
Coupons change the arithmetic and are worth checking, with one caveat: a coupon for a product you would not otherwise buy is a discount on a purchase you did not need. The honest comparison is unit price after the coupon against the unit price of what you already buy.
The habit costs one division per package, and it survives because it is a rule rather than a project. Divide, compare, buy the cheaper unit unless there is a reason not to — and when there is a reason, write it down. After a month, the list of reasons is short, and the savings are the difference between the two prices on everything else.
Many shelf labels already carry a unit price, usually in small type in a corner. Use it, but check it once per aisle against your own division. Label errors happen, and a label computed per ounce when the package is sold by weight will be wrong in a way that looks authoritative.
Labels also sometimes use a base you would not choose. Per ounce is standard for dry goods, per 100 grams for imported products, per fluid ounce for liquids and per square foot for paper goods. Comparing a per-ounce label to a per-100-gram label produces nonsense, which is why normalising both to the same base is the step worth doing by hand when the units differ.
Online grocery listings are worse, because the unit price may be computed per item rather than per unit of product. A multipack listed with a per-item price and a single item listed with a per-ounce price are not comparable until you convert one of them.
Last updated: November 2, 2026. Estimates only. Not tax, legal, financial, or professional advice. See the terms.