Unit Price Calculator
This unit price calculator compares up to four items at once and gives the price per gram, ounce, milliliter, sheet or tablet for each one. It marks the cheapest and shows how much more the others cost as a percentage.
| Item | Price | Size / quantity |
|---|---|---|
How to use the unit price calculator
- Set the unit first. Type the unit you want to compare in — g, oz, ml, fl oz, sheet, tablet, load, diaper. Everything you enter afterwards has to be expressed in that same unit.
- Enter each item. Give it a name so the chart is readable, then the price and the size or quantity. Two items is enough; four is the maximum.
- Read the per-unit column. Results appear as you type. The cheapest item is marked, and each of the others carries a percentage showing how much more it costs per unit than the winner.
- Check the chart. The bars are drawn to scale, so a difference that looks trivial in cents per gram shows up as an obvious gap.
Name the items properly. “Store brand 1kg” is easier to act on at the shelf than “Item 1”.
The unit price formula
The whole method rests on one rule: the divisor has to mean the same thing for every item you are comparing. Grams against grams, sheets against sheets. A price per gram and a price per ounce look like comparable numbers and are not, because an ounce is more than 28 times a gram.
Two things follow from the formula:
- Lower is better. The unit price is a cost, so the smallest number wins.
- The percentage gap is computed against the cheapest item, using
(this unit price − cheapest unit price) ÷ cheapest unit price × 100. An item at $0.90 per 100 g against a winner at $0.75 is 20% more expensive, not 15% more.
If you would rather see the same division worked out step by step on its own, the division calculator shows the long division.
Worked examples
Example: two boxes of the same cereal
Small box: 350 g for $3.29. Large box: 750 g for $7.49. Set the unit to g and compare per 100 g.
The big box is the more expensive one, by 6.2% per 100 g. In dollars, 750 g bought at the small box’s rate would cost $7.05, so the large box charges $0.44 more for the same amount of cereal.
Example: an ounce price against a gram price
Item A is 12 oz for $4.29. Item B is 500 g for $5.99. Convert A to grams first.
B is cheaper by 5.3% per 100 g, even though it carries the higher shelf price. Comparing $4.29 with $5.99 tells you nothing on its own.
Example: rolls against sheets
Paper towels sold as 12 rolls of 160 sheets for $11.99, against 9 rolls of 220 sheets for $12.49. Set the unit to sheet.
Pack 1 wins by about 1%. The pack with fewer rolls has more sheets, which is exactly the kind of comparison that is impossible to eyeball.
Comparing items sold in different units
Manufacturers rarely use the same unit for competing products, and that is often the point. Convert everything to one unit before dividing. These are the factors you need.
| From | To | Multiply by |
|---|---|---|
| 1 ounce (oz) | grams | 28.3495 |
| 1 pound (lb) | grams | 453.592 |
| 1 pound (lb) | ounces | 16 |
| 1 kilogram (kg) | pounds | 2.20462 |
| 1 fluid ounce (US fl oz) | milliliters | 29.5735 |
| 1 US pint | milliliters | 473.176 |
| 1 US quart | milliliters | 946.353 |
| 1 US gallon | liters | 3.78541 |
| 1 liter | US fluid ounces | 33.814 |
A second trap is the imperial fluid ounce used outside the US, which is 28.4131 ml rather than 29.5735 ml. If a product was packaged for a different market, check which one the label means before converting.
Why price per 100 g is the useful standard
Supermarket shelf labels usually quote price per 100 g or per 100 ml for food, and per liter or per kilogram for bulk goods. There is nothing magic about 100 — it just puts the answer in a range you can read. A cost of $0.0119 per gram is hard to hold in your head; $1.19 per 100 g is not, and it lines up neatly against the shelf tag when you want to check it.
Pick one basis and stay on it for the whole comparison. Per 100 g works for food, per liter for drinks and detergent, per sheet for paper, per tablet for medicine, per load for laundry products that state a load count.
The bigger box is not always cheaper
Buying the larger size is a reliable habit, not a reliable saving. The cereal example above has the 750 g box costing 6.2% more per 100 g than the 350 g box. That pattern shows up often enough that it is worth checking every time, and there are ordinary reasons for it:
- The small size is the one running a promotion this week.
- The large size is priced on the assumption that shoppers do not check.
- The large size comes in packaging that costs more per gram, such as a resealable pouch.
Unit price also is not the last word. A larger pack you will not finish before it spoils has a real cost per gram far above the shelf calculation, because part of what you bought goes in the trash. If you throw away a quarter of a $8.00 bulk bag, you effectively paid $8.00 for three quarters of it, which is a 33% higher price per usable gram.
Multi-buys, 3 for 2 and bulk offers
Offers stated as a deal rather than a price have to be flattened into a single price and a single quantity before they can be compared. The rule is: enter the total you actually hand over, and the total quantity you actually take home.
Example: 3 for 2 on a $4.50 pack
Each pack is 400 g. You take three and pay for two.
The offer is a 33.3% discount, because you pay for two thirds of what you carry out. Against a rival 1 kg bag at $8.20, which is $0.82 per 100 g, the 3 for 2 wins.
Example: 2 for $7 against $3.99 each
Worth taking if you want two. If you only want one, the offer is worth nothing, and the item still costs $3.99.
The same flattening handles the rest of them:
- Buy one get one free — price stays the same, quantity doubles, so the unit price halves. That is 50% off, not 100%.
- Buy two get the third half price — on a $6 item you pay 6 + 6 + 3 = $15 for three, which is $5 each, a 16.7% discount.
- Buy one get one 50% off — you pay $9 for two $6 items, $4.50 each, a 25% discount.
If you are the one setting these prices rather than reading them, the markup calculator shows what each of these offers leaves in the margin.
Common mistakes
- Mixing units across items. Convert to a single unit before dividing. 12 oz is 340.19 g, not 12 g.
- Treating fluid ounces as ounces. One is volume, one is weight, and there is no conversion between them.
- Comparing shelf prices. A higher shelf price often carries a lower unit price, and the reverse is just as common.
- Entering the pack price for a multi-buy. Enter the total paid and the total quantity, not the price of one pack.
- Forgetting the pack count. A carton of 6 × 330 ml is 1,980 ml, not 330 ml.
- Comparing drained weight against net weight. Canned goods list both, and only one of them is what you eat.
- Using the ticket price on a discounted item. Take the discount off first.
- Dividing the percentage gap by the wrong item. The gap is measured against the cheapest unit price, not against the item you were going to buy anyway.
Frequently asked questions
How do I compare a 12 oz pack with a 500 g pack?
Convert the ounces to grams, then divide. 12 oz × 28.3495 = 340.19 g. If the 12 oz pack is $4.29 that is $1.26 per 100 g. If the 500 g pack is $5.99 that is 5.99 ÷ 5 = $1.20 per 100 g, so the metric pack is cheaper by 5.3% despite the higher shelf price.
Which is cheaper: 350 g for $3.29 or 750 g for $7.49?
The small box. 3.29 ÷ 350 = $0.94 per 100 g, while 7.49 ÷ 750 = $1.00 per 100 g. The larger box costs 6.2% more per gram. It is a good illustration that pack size and value move independently, and that the bigger box needs checking rather than assuming.
What is price per 100 g and why do stores use it?
It is the cost of 100 grams of the product: price ÷ grams × 100. Stores use it because it gives a number in a readable range for most groceries. Per-gram prices run to four decimal places, which is hard to compare at a glance, while $1.19 per 100 g lines up directly against the shelf tag.
Is the bigger size always cheaper per unit?
No. Larger packs are often better value but not reliably so, because promotions, packaging costs and pricing decisions all cut across size. Check every time. Also weigh whether you will use it: a bulk pack you throw a quarter of away costs 33% more per gram you actually consume than the shelf math suggests.
How do I work out a 3 for 2 offer?
Enter the total you pay and the total you receive. Three $4.50 packs of 400 g on 3 for 2 costs 2 × 4.50 = $9.00 for 1,200 g, which is $0.75 per 100 g. Against $1.125 per 100 g at the single price, that is a 33.3% discount — you pay for two thirds of what you take home.
How many milliliters are in a fluid ounce?
A US fluid ounce is 29.5735 ml, so a 16 fl oz bottle holds 473.2 ml. The imperial fluid ounce used in the UK and elsewhere is 28.4131 ml, which is why a “20 fl oz” imperial pint is 568 ml. Check which system the label uses before converting, especially on imported products.
What unit should I use for paper towels or diapers?
Use the countable thing, not the pack. For paper towels that is sheets: 12 rolls of 160 sheets is 1,920 sheets. For diapers it is the diaper count. Comparing per roll or per pack hides the fact that roll sizes and pack sizes differ between brands, which is usually where the difference in value sits.
Does the lowest unit price always mean the best buy?
It is the best price per unit, which is not always the best purchase. Spoilage, storage, whether you will use it, and the cash tied up all matter. Unit price answers one question exactly: what a gram, sheet or tablet costs from each option. Decide the rest yourself with that number in front of you.
How do I compare a buy one get one free offer?
The price stays the same and the quantity doubles, so the unit price halves. That is exactly 50% off, not 100% off. Enter the price of one pack and twice the pack size. Buy one get one half price is milder: two $6 items for $9 is $4.50 each, a 25% discount.
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