Match the product and the unit, include every pack in a multipack, and check the conditions attached to a discount. Compare the unrounded ratios before deciding whether a displayed tie is real. Unit price describes cost per quantity—not quality, suitability or how much you need.
Choose one unit before comparing the numbers
Begin with comparable goods: the same useful product type, the same currency and the same price basis. Price per 100 g cannot be compared directly with price per kilogram. Convert one side first. The NIST unit-pricing guide recommends consistent measurement units within a product category so pack sizes can be compared meaningfully.
For mass, 1 kg = 1,000 g. For volume, 1 L = 1,000 mL. A price per kilogram is ten times the price per 100 g; dividing it by ten puts it on the same basis. Keep the quantity written beside every calculation so you do not compare two unlabeled decimals.
If a 600 g pack costs an invented $3.30, its price per 100 g is $3.30 ÷ 600 × 100 = $0.55. The price per kilogram is $5.50. Those are two ways to express the same ratio, not different deals.
Do not treat grams and millilitres as interchangeable: one measures mass and the other volume. A “fl oz” label is also not the same as an “oz” weight label. Choose compatible units or get the information needed to make a valid conversion. For count-based comparisons, check that the items are equivalent rather than assuming every roll, tablet or notebook represents the same amount of useful product.
Compare a single pack, a larger pack and a multipack
Every price below is invented for this worked example. Assume equivalent contents, no extra charges and no other discounts. The three checkout totals buy different quantities, so comparing only $3.20, $4.50 and $5.20 does not answer which costs less per 100 g.
| Option | Quantity received | Checkout total | Unit-price calculation | Price per 100 g |
|---|---|---|---|---|
| A · Single 500 g pack | 1 × 500 g = 500 g | $3.20 | $3.20 ÷ 500 × 100 | $0.64 |
| B · Single 750 g pack | 1 × 750 g = 750 g | $4.50 | $4.50 ÷ 750 × 100 | $0.60 |
| C · Two 400 g packs | 2 × 400 g = 800 g | $5.20 | $5.20 ÷ 800 × 100 | $0.65 |
Same comparison unit. Different checkout totals.
Each full track represents $1.00 per 100 g. A shorter bar means a lower unit price.
- A · Single 500 g pack$0.64 / 100 g$3.20 checkout · 500 g total
- B · Single 750 g pack$0.60 / 100 g$4.50 checkout · 750 g total
- C · Two 400 g packs$0.65 / 100 g$5.20 checkout · 800 g total
B is lowest per 100 g at $0.60. A is $0.64 and C is $0.65. The multipack contains 800 g, not 400 g, so its full $5.20 price belongs over 800 g. A common error is to divide the multipack price by one inner pack and accidentally double its unit price.
A still has the lowest checkout total: $3.20. If you need only 500 g, B requires another $1.30 today and supplies 250 g more. The ratio does not decide whether you will use the extra quantity, have room to store it or want that particular product. The ACCC’s grocery unit-price explanation similarly separates the total selling price from a standard-unit comparison.
Use the offer price only when its conditions apply
Read what quantity and eligibility produce the advertised total. A stated “two for $4.80” deal does not by itself mean one costs $2.40. Check the single-item price and whether the offer requires both items, a membership, a coupon or another condition. Do not subtract a possible future reward from today’s payment as though it were an immediate discount.
For the original example in question 5, two 300 g packs together cost $4.80, but one alone costs $2.70. The two-pack calculation is $4.80 ÷ 600 × 100 = $0.80 per 100 g. Buying one uses a different ratio: $2.70 ÷ 300 × 100 = $0.90 per 100 g. Which price applies is part of the problem, not a detail to fill in after division.
If required charges differ between the options, make their treatment explicit before comparing. Keep separate currencies, refundable deposits and conditional rewards out of an unlabeled total. This guide teaches the arithmetic; it does not describe every retailer’s policy or the unit-price display rules in every country.
Carry enough decimal places through the calculation. For close results, compare the original fractions or use cross-multiplication: with positive quantities, compare price A × quantity B with price B × quantity A. The fraction comparison guide explains why using the same denominator matters. Round only the displayed result; a rounded tie may hide unequal ratios.
Try six original questions with worked answers
Write a comparison unit before opening an answer. Use total quantities, retain the checkout price and state any offer condition. The $ prices are fictional arithmetic inputs, not current deals, recommended purchases or a claim that one product is better for everyone.
1. Compare 300 g for $1.80 with 500 g for $2.75. Which costs less per 100 g?
The 500 g pack: $0.55 per 100 g. The 300 g pack costs $1.80 ÷ 300 × 100 = $0.60 per 100 g. The 500 g pack costs $2.75 ÷ 500 × 100 = $0.55. Its unit price is lower, although its checkout price is $0.95 higher.
2. Compare three 250 g packs for $4.50 total with one 1 kg pack for $6.40. Which has the lower unit price?
The three-pack: $0.60 per 100 g. Its total quantity is 3 × 250 = 750 g, so $4.50 ÷ 750 × 100 = $0.60. Convert 1 kg to 1,000 g: $6.40 ÷ 1,000 × 100 = $0.64. Do not divide the three-pack’s total price by only one 250 g pack.
3. The same drink costs $2.10 for 1.5 L or $1.20 for 750 mL. Compare the price per 100 mL.
The 1.5 L bottle costs less per 100 mL: $0.14 versus $0.16. First convert 1.5 L to 1,500 mL. Then calculate $2.10 ÷ 1,500 × 100 = $0.14 and $1.20 ÷ 750 × 100 = $0.16. The smaller bottle still needs less money at checkout.
4. Six identical notebooks cost $4.50 and ten cost $8.00. Which pack is cheaper per notebook?
The six-pack: $0.75 per notebook. $4.50 ÷ 6 = $0.75, while $8.00 ÷ 10 = $0.80. The count comparison assumes the notebooks are equivalent; a different size or page count may change what is useful to compare.
5. A stated offer is two 300 g packs for $4.80 total; one alone is $2.70. A 500 g pack is $4.25. What changes if you buy only one offer pack?
Buying both offer packs gives $0.80 per 100 g; buying one gives $0.90. Both: $4.80 ÷ 600 × 100 = $0.80. One: $2.70 ÷ 300 × 100 = $0.90. The 500 g alternative costs $4.25 ÷ 500 × 100 = $0.85. The offer is lower per unit only when its stated two-pack condition is met; it also requires a $4.80 checkout payment.
6. Compare 400 g for $2.01 with 500 g for $2.51. Both unit prices display as $0.50 per 100 g to two decimal places. Are their exact ratios equal?
No. The 500 g pack has the slightly lower exact ratio. $2.01 ÷ 400 × 100 = $0.5025 per 100 g; $2.51 ÷ 500 × 100 = $0.5020. Each rounds to $0.50, but rounding hides the difference. This tiny unit-price gap does not change the fact that the smaller pack costs $0.50 less at checkout.
Use a short check before choosing
- Match: comparable contents, currency and measurement type.
- Total: the price actually payable and the full quantity received under the applicable conditions.
- Normalize: express both ratios per the same 100 g, 100 mL, kilogram, litre or equivalent item.
- Separate: lower cost per unit, lower checkout payment and the quantity you need are different questions.
Estimate the scale before calculating exactly. If a 500 g pack is about half a kilogram, a $3 price should work out near $6 per kilogram—not $60. Use mental multiplication shortcuts to check simple scaling, then keep exact figures for close comparisons.
For more arithmetic practice, try the adult mental-math routine. The separate Math Riddles app offers number and logic puzzles, hints and solutions, with live 1v1 play described in its official listings. It is available for iPhone, iPad and Android, with ads and optional in-app purchases; check the current store for feature access.
These examples are website exercises, not app levels or a verified shopping feature. No answers or shopping inputs are collected here. A correct calculation does not guarantee a better purchase or any particular savings.
Common questions
How do I calculate price per 100 g?
Divide the total price by the total weight in grams, then multiply by 100. For an invented $3.30 price and 600 g weight, $3.30 ÷ 600 × 100 = $0.55 per 100 g.
Is the largest pack always the cheapest?
No. Calculate each unit price using the same basis. In the fictional comparison, the 750 g pack is cheaper per 100 g than either the 500 g pack or the 800 g multipack. The smaller 500 g pack still has the lowest checkout total.
How do I work out a multipack unit price?
Multiply the number of equal inner packs by the quantity in each, then divide the total multipack price by that full quantity. For unequal inner packs, add their comparable quantities instead. Apply the same comparison unit to both options.
Can I compare price per kilogram with price per 100 g?
Yes, after converting to the same basis. Divide price per kilogram by ten to obtain price per 100 g, or multiply price per 100 g by ten to obtain price per kilogram.
Does a lower unit price mean I pay less at checkout?
Not necessarily. A larger quantity can have a lower cost per gram but a higher total price. Keep quantity needed, product suitability, potential waste and the actual payment separate from the unit-price calculation.
References and further reading
Published by Moocsoft, the independent studio behind Math Riddles. Examples and worksheets are illustrative. App features can vary by platform and version; see the store listing for current availability and in-app purchases.