Answer first
What this calculator tells you
Compare the unit price of two package sizes and see how much the cheaper one saves. Find out whether the larger pack is really the better buy. Formula: Unit price = price ÷ quantity; saving = (higher unit price − lower unit price) ÷ higher unit price. At the worked-example inputs, the unit price of item a is 0.299. Holding every other input steady, moving price of item a from $3.83 to $5.75 moves the result from 0.239 to 0.359.
Transparent method
The formula
Find out whether the larger pack is really the better buy.
Worked example
Example inputs
How to interpret the result
The bigger pack is not always the cheaper one. Compare price divided by quantity, in the same unit. A 16 ounce jar at $4.79 costs about 29.9 cents an ounce, and a 32 ounce jar at $8.99 costs about 28.1, so the large one saves roughly 6 percent. The gap can be small, and sometimes a sale on the small size beats the bulk price outright.
At the worked-example inputs the unit price of item a is 0.299. It rises with price of item a and falls as quantity in item a increases; price of item b and quantity in item b do not move it.
This educational estimate omits individual tax and legal considerations.
Before you rely on it
What to check
Make sure both quantities use the same unit. Weight in ounces against volume in fluid ounces, or against a count of sheets, gives a comparison that is not real.
The common error
Where people go wrong with unit price comparison calculator
Buying the big size and letting part of it spoil. A saving of 6 percent disappears if you throw away a tenth of the product.
Sensitivity evidence
How price of item a changes the unit price of item a
Holding every other input at the worked-example value, moving price of item a from $3.83 to $5.75 moves the unit price of item a from 0.239 to 0.359: a spread of 0.12, or 40% of the worked-example result.
| Price of item A | Unit price of item A | Unit price of item B | Saving from the cheaper item |
|---|---|---|---|
| $3.83 | 0.239 | 0.281 | 14.8% |
| $4.31 | 0.269 | 0.281 | 4.1% |
| $4.79worked example | 0.299 | 0.281 | 6.2% |
| $5.27 | 0.329 | 0.281 | 14.7% |
| $5.75 | 0.359 | 0.281 | 21.8% |
Every input, tested
Which input moves the unit price of item a most
Of the 4 inputs, quantity in item a moves the unit price of item a most (0.076 across the range tested) and price of item a moves it least (0.06). Price of item b and quantity in item b do not change it at all.
| Input | Tested from | To | Unit price of item A at each end | Swing |
|---|---|---|---|---|
| Quantity in item A | 14 | 18 | 0.342 to 0.266 | 0.076 (25%) |
| Price of item A | $4.31 | $5.27 | 0.269 to 0.329 | 0.06 (20%) |
| Price of item B | $8.00 | $10 | 0.299 to 0.299 | none |
| Quantity in item B | 29 | 35 | 0.299 to 0.299 | none |
Two variables at once
Unit price of item A by price of item a and quantity in item a
Across the grid the unit price of item a runs from 0.202 to 0.442. Moving price of item a from $3.83 to $5.75 shifts it by 0.12 at the middle column, and moving quantity in item a from 13 to 19 shifts it by 0.116 at the middle row, so price of item a is the bigger lever here.
| Price of item A \ Quantity in item A | 13 | 16 | 19 |
|---|---|---|---|
| $3.83 | 0.295 | 0.239 | 0.202 |
| $4.31 | 0.332 | 0.269 | 0.227 |
| $4.79 | 0.368 | 0.299 | 0.252 |
| $5.27 | 0.405 | 0.329 | 0.277 |
| $5.75 | 0.442 | 0.359 | 0.303 |
The highlighted cell is the worked example: 0.299.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Price of item A | $4.79 | Enter the price of item a used in this calculation. |
| Quantity in item A | 16 | Ounces, grams, sheets or any unit. Use the same unit for item B. |
| Price of item B | $8.99 | Enter the price of item b used in this calculation. |
| Quantity in item B | 32 | Enter the quantity in item b used in this calculation. |
| Unit price of item A | 0.299 | |
| Unit price of item B | 0.281 | |
| Saving from the cheaper item | 6.2% | |
Inputs, definitions and assumptions
Price of item A
Enter the price of item a used in this calculation. The prefilled worked-example value is $4.79.
Quantity in item A
Ounces, grams, sheets or any unit. Use the same unit for item B. The prefilled worked-example value is 16.
Price of item B
Enter the price of item b used in this calculation. The prefilled worked-example value is $8.99.
Quantity in item B
Enter the quantity in item b used in this calculation. The prefilled worked-example value is 32.
How to use this calculator
- 1Verify the inputs. Gather price of item a, quantity in item a, price of item b and quantity in item b from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the unit price of item a at 0.299. Store your own version of it as Scenario A.
- 3Test one change. Start with quantity in item a, the input with the biggest effect here: moving quantity in item a from 14 to 18 takes the unit price of item a from 0.342 to 0.266, a swing of 25% of the worked-example figure.
- 4Check the extremes. At half the example quantity in item a (8) the unit price of item a is 0.599; at double (32) it is 0.15.
People also ask
Frequently asked questions
How do you calculate unit price comparison?
Unit price = price ÷ quantity; saving = (higher unit price − lower unit price) ÷ higher unit price. Enter price of item a in dollars and price of item b in dollars. At the worked-example inputs the unit price of item a is 0.299.
What does the unit price comparison result mean?
Find out whether the larger pack is really the better buy. At the worked-example inputs the unit price of item a is 0.299. It rises with price of item a and falls as quantity in item a increases; price of item b and quantity in item b do not move it.
How much does price of item a change the unit price of item a?
Holding every other input at the worked-example value, moving price of item a from $3.83 to $5.75 moves the unit price of item a from 0.239 to 0.359, a spread of 0.12.
What are the limits of this unit price comparison calculator?
This educational estimate omits individual tax and legal considerations. The tables on this page test price of item a only from $3.83 to $5.75; a value outside that range is not tabulated here.
Which input moves the unit price of item a most in the unit price comparison calculator?
Ranked by how far each moves the unit price of item a across the range tested: quantity in item a (0.076, 25%) and price of item a (0.06, 20%). Price of item b and quantity in item b do not change it.
If I double quantity in item a in the unit price comparison calculator, does the unit price of item a double?
Doubling it from 16 to 32 takes the unit price of item a from 0.299 to 0.15, which is 0.50 times the worked-example figure. So it falls instead of rising. Halving it to 8 gives 0.599.
How much does quantity in item a matter in the unit price comparison calculator?
The worked example uses 16. Holding every other input at its worked-example value, moving quantity in item a from 14 to 18 takes the unit price of item a from 0.342 to 0.266, a swing of 25% of the worked-example figure.
How much does price of item b matter in the unit price comparison calculator?
The worked example uses $8.99. The unit price of item a does not depend on price of item b; it moves the unit price of item b from 0.25 to 0.313 instead when price of item b goes from $8.00 to $10.
How much does quantity in item b matter in the unit price comparison calculator?
The worked example uses 32. The unit price of item a does not depend on quantity in item b; it moves the unit price of item b from 0.31 to 0.257 instead when quantity in item b goes from 29 to 35.
Which inputs change the unit price of item b in the unit price comparison calculator?
At the worked-example inputs it is 0.281. Price of item b takes it from 0.25 to 0.313 and quantity in item b takes it from 0.31 to 0.257.
Which inputs change the saving from the cheaper item in the unit price comparison calculator?
At the worked-example inputs it is 6.2%. Price of item a takes it from 4.1% to 14.7%, quantity in item a takes it from 17.9% to 5.3%, price of item b takes it from 16.5% to 4.2% and quantity in item b takes it from 3.4% to 14.2%.
How does inflation affect what my money buys?
It reduces purchasing power at a compounding rate. So a fixed amount buys steadily less over time. Over long horizons the effect is large, which is why planning that mixes today's costs with future nominal returns systematically overstates what will actually be affordable.
What is the rule of 72?
Dividing 72 by a percentage rate approximates the years for a balance to double. It is accurate enough for mental arithmetic in the middle of the range and drifts at the extremes. It is a sanity check , not a calculation.
Sources and evidence
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