How to find the best unit price
Grocery shelves display dozens of sizes and prices, and bigger packages often look like the obvious deal. They usually are — but not always. Unit price is the one number that cuts through all the packaging and promotions and lets you compare any two products directly.
The formula: price ÷ size
Unit price = total price ÷ total quantity
The result tells you how much you pay per single unit — per ounce, per gram, per count, per fluid ounce, or whatever unit the product is sold in. The lower the unit price, the better the deal, assuming the quality is equal.
Example: A 16 oz jar of peanut butter costs $3.84. What is the unit price per ounce?
$3.84 ÷ 16 oz = $0.24 per oz
A 40 oz jar of the same brand costs $8.80.
$8.80 ÷ 40 oz = $0.22 per oz
The big jar is cheaper per ounce — by $0.02/oz. Over 40 oz, that's $0.80 in savings. If you go through peanut butter regularly, the larger jar wins. If half of it would go to waste, the smaller jar may cost less in practice.
The units problem: comparing apples to apples
Unit price only works when both products are measured in the same unit. When they're not, you have to convert first.
Common conversions to know
- 1 pound (lb) = 16 ounces (oz)
- 1 kilogram (kg) = 1,000 grams (g)
- 1 fluid ounce (fl oz) = approximately 29.57 milliliters (mL)
- 1 liter (L) = 1,000 mL = approximately 33.81 fl oz
The cleanest approach: pick one target unit and convert everything to it before dividing. For food sold by weight, grams are convenient because the numbers stay whole. For liquids, milliliters work the same way.
Example: You want to compare two bags of rice:
- Bag A: 2 lb for $2.79
- Bag B: 900 g for $2.49
Convert both to grams. Bag A: 2 lb = 2 × 453.59 g = 907.18 g.
- Bag A: $2.79 ÷ 907.18 g = $0.00307 per gram
- Bag B: $2.49 ÷ 900 g = $0.00277 per gram
Bag B is cheaper by about $0.03 per 100 g — a small difference on one bag, but it adds up over a household's weekly shop.
Worked grocery comparison: paper towels
Paper towels often come in a range of pack sizes with varying sheet counts and prices. Here's a real-style comparison:
| Pack | Total price | Total sheets | Price per 100 sheets |
|---|---|---|---|
| 6 single rolls (120 sheets each = 720 sheets) | $7.99 | 720 | $1.11 |
| 8 double rolls (240 sheets each = 1,920 sheets) | $14.49 | 1,920 | $0.75 |
| 12 double rolls (240 sheets each = 2,880 sheets) | $19.99 | 2,880 | $0.69 — best value |
| 6 triple rolls (360 sheets each = 2,160 sheets) | $17.99 | 2,160 | $0.83 |
The 12 double rolls win at $0.69 per 100 sheets. The 6 triple rolls are actually more expensive per sheet than the 8 double rolls, even though they look like a bulk deal. This is exactly the kind of comparison that looks obvious in a table but is easy to get wrong eyeballing the shelf.
Why the bigger size isn't always cheaper
Several situations make larger packages worse deals than they appear:
- Sale on the smaller size. A temporary discount on the small size can make it cheaper per unit than the everyday-priced large. Check the shelf tag date, or calculate unit price on each size independently before assuming the bulk package wins.
- "New" sizes with hidden shrinkflation. Manufacturers sometimes reduce package contents while keeping the price — or the physical package — the same size. If the price stayed flat but the net weight dropped, the unit price went up. The number on the nutrition label or the fine print on the package tells you the actual quantity.
- Store brand vs name brand. A name-brand large size might still have a higher unit price than a store-brand small size. Always calculate both, not just within a brand.
- Buy-one-get-one (BOGO) deals. A BOGO is effectively a 50% discount on the pair. To compare with a regular price on a different size, use: effective unit price = total price for both ÷ total quantity of both. Example: two 18 oz containers at $5.49 each (BOGO) = $5.49 for 36 oz = $0.15 per oz.
- Waste threshold. If a bulk size will expire or spoil before you use it, the savings are illusory. A unit price comparison assumes you'll actually use everything you buy.
Per-100g vs per-oz: which to use
For most purposes, either works — you're just picking a denominator. Per-100g is convenient because it gives a number in the cents-to-dollars range for most grocery staples, making comparisons easy to read at a glance. Per-oz is more natural if you're already thinking in U.S. customary units and the shelf tags use ounces.
The only rule that matters: use the same denominator for everything you're comparing. Mixing per-oz and per-100g numbers in the same comparison will give wrong results.