Two bottles of ketchup. One is 20 oz for $3.49. The other is 32 oz for $4.99. Which is the better deal?
Most people grab the bigger bottle assuming it’s cheaper. But let’s check: the 20 oz bottle is $0.175 per ounce, while the 32 oz bottle is $0.156 per ounce. The bigger bottle wins — but only by about 2 cents per ounce.
Now add a wrinkle: the 20 oz bottle has a “buy one get one 50% off” promotion. With the deal, two 20 oz bottles (40 oz total) cost $5.24, or $0.131 per ounce — significantly cheaper than the single 32 oz bottle.
This is why unit price matters. The deal that looks obvious often isn’t.
How to Calculate Unit Price
The formula is simple: Unit Price = Total Price ÷ Total Quantity.
$4.99 for 32 oz: $4.99 ÷ 32 = $0.156 per ounce. $7.99 for 64 oz: $7.99 ÷ 64 = $0.125 per ounce. $2.99 for 12 oz: $2.99 ÷ 12 = $0.249 per ounce.
The lowest unit price is the best value, assuming you’ll actually use the full amount before it expires.
Common Shopping Traps
The “family size” assumption. Family and bulk sizes are usually cheaper per unit, but not always. During sales, regular sizes with promotions frequently beat bulk prices. Check every time.
Different unit measurements. One brand lists price per ounce, another lists price per 100 grams. You can’t compare directly without converting. Our Unit Price Calculator handles this automatically.
Ignoring expiration dates. A gallon of milk at $0.04/oz beats a half gallon at $0.05/oz — unless you can’t drink a full gallon before it spoils. The “savings” disappear if you throw away 30% of the product.
Multi-buy deals. “3 for $10” sounds great, but if the regular price is $3.49 each, you’re only saving $0.49 total — and you might not need three. Worse, some stores set the “3 for $10” price at exactly $3.33 each, meaning there’s no savings at all.
Where Unit Price Saves the Most
The biggest savings come from items you buy frequently. Even small per-unit differences add up dramatically over a year.
Cereal: Premium brands can cost $0.35/oz while store brands run $0.15/oz. On 2 boxes per month, switching saves about $60/year.
Beverages: Single bottles cost $0.05-$0.10/oz while multi-packs run $0.02-$0.04/oz. A family drinking 5 bottles per week saves $100+/year.
Cleaning supplies: Concentrated formulas at $0.10/oz dilute to effectively $0.02/oz, while ready-to-use versions cost $0.06/oz. Smart concentrate buying saves $30-$50/year.
Paper products: Toilet paper ranges from $0.03 to $0.08 per square foot depending on brand, size, and deals. Consistent unit price shopping saves $40-$70/year.
The Costco Question
Is a warehouse membership worth it? The math depends on what you buy.
For non-perishable staples, cleaning supplies, and paper products, Costco’s unit prices are typically 20-40% below grocery stores. For a family spending $200/month on these categories, the savings easily cover the $65 annual membership.
But warehouse shopping has traps: impulse purchases on items you wouldn’t normally buy, perishable bulk food that goes to waste, and the $1.50 hot dog combo that somehow turns every trip into a $300 expedition.
Start Saving Today
Use our free Unit Price Calculator to quickly compare any two (or more) products side by side. Just enter the price and quantity of each option, and instantly see which is the better deal per unit.