A big yellow sticker saying "Save 30%" tells you almost nothing about what a multipack costs per gram, per millilitre or per can. The shelf label might help, but it is often printed in a different unit from the pack next to it, and it never includes the coupon you need a minimum basket for, or the delivery fee that only appears at the last step.
This guide shows how to turn any Black Friday or Cyber Monday offer into one honest number. Every example uses made-up prices chosen so the arithmetic is easy to repeat on paper. None of them is a recommendation of a shop or a brand; the point is the method, which works on any offer you meet.
The method in one line
For each offer, work out what you pay at checkout and how much you receive, then divide one by the other and scale it to a standard amount: 100 g, 100 ml or one item. Compare those numbers only among offers measured in the same kind of thing.
That line hides six small steps. Each one below fixes one way a deal can look better than it is.
Step 1: put every pack in one unit
Offers arrive in kilograms, pounds, ounces, litres, millilitres and fluid ounces, sometimes all on the same page. Convert everything to grams or millilitres first, then divide.
| Unit | Equals |
|---|---|
| 1 kg | 1,000 g |
| 1 lb | 453.59 g |
| 1 oz | 28.35 g |
| 1 L | 1,000 ml |
| 1 US fl oz | 29.57 ml |
| 1 dozen | 12 items |
Take three bags of oat flakes. The prices are invented, the conversions are real.
| Offer | Total | Weight in grams | Price per 100 g |
|---|---|---|---|
| 1.2 kg bag | 3.60 | 1,200 | 0.30 |
| 500 g box | 1.75 | 500 | 0.35 |
| 2 lb tub | 3.99 | 907.18 | 0.44 |
The tub looks cheapest because 3.99 is the smallest number, yet it costs nearly half as much again per 100 g as the bag. Dividing by the weight in grams, not by the pack count, is what exposes it.
Volume works the same way. A multipack of six 330 ml cans holds 1,980 ml. At 4.95 that is 0.25 per 100 ml. A 1.5 L bottle at 3.30 is 0.22 per 100 ml. A 33 fl oz bottle at 2.60 holds about 975.87 ml and comes to 0.27 per 100 ml. Three different units, one clear order.
Never compare across kinds. 500 ml of cooking oil and 500 g of rice have no honest common price, so weight, volume and count offers are ranked in separate lists.
Step 2: count the free packs
"Buy 3, get 1 free" is a quantity offer in disguise. You pay for three packs and receive four, so divide the money by four packs, not three. Write it as paid packs and free packs, and the sum does itself.
| Deal | You pay | You get | Price per 100 g |
|---|---|---|---|
| Buy 3 get 1 free, 250 g jars at 2.50 each | 7.50 | 1,000 g | 0.75 |
| One 800 g jar | 5.60 | 800 g | 0.70 |
| Buy 2 get 1 free, 400 g packs at 3.20 each | 6.40 | 1,200 g | 0.53 |
| One 400 g pack | 3.20 | 400 g | 0.80 |
Two things stand out. The first deal sounds generous, yet the plain 800 g jar beats it by 0.05 per 100 g. The third deal is a genuine saving: the free pack cuts the price by a third compared with buying one pack alone. Without the division you cannot tell which is which.
Step 3: apply the coupon, and respect the minimum
A basket coupon usually has a minimum spend, and the minimum is measured on the items subtotal before delivery. The rule to remember: the coupon either applies in full or not at all. There is no partial credit for being close.
Suppose a coupon gives 15% off when you spend 20.00 or more.
| Items subtotal | Coupon applies? | Discount | You pay |
|---|---|---|---|
| 19.00 | No, 1.00 short | 0.00 | 19.00 |
| 19.99 | No, 0.01 short | 0.00 | 19.99 |
| 20.00 | Yes, exactly at the minimum | 3.00 | 17.00 |
| 21.50 | Yes | 3.23 | 18.27 |
Look at the first and last rows. Adding a 2.50 item to a 19.00 basket lowers what you pay from 19.00 to 18.27, so the extra item is effectively free and you leave with more. That only works when you would use the extra item anyway. If it would sit in a cupboard until next year, the saving is imaginary.
The 21.50 row also shows how a percentage is rounded: 15% of 21.50 is 3.225, which becomes 3.23 because a half rounds up to the next cent. A fixed-amount coupon is simpler. It can never take more than the subtotal, and it is never taken off delivery.
Step 4: add delivery, then watch the winner change
Delivery is charged once per order, however many packs you buy. That single fixed cost is why the best offer for a small order is often a different offer from the best for a big one.
Take two offers for the same 2 kg of the same product. Offer X is 11.00 plus 4.50 delivery. Offer Y is 12.40 with free delivery. On the shelf, X is 0.55 per 100 g and Y is 0.62, so X looks better. Now count the delivery as you buy more whole offers.
| You need | Offer X: offers × 11.00 + 4.50 | Offer Y: offers × 12.40 | Cheaper |
|---|---|---|---|
| 2 kg (1 offer) | 15.50 | 12.40 | Y by 3.10 |
| 6 kg (3 offers) | 37.50 | 37.20 | Y by 0.30 |
| 8 kg (4 offers) | 48.50 | 49.60 | X by 1.10 |
| 10 kg (5 offers) | 59.50 | 62.00 | X by 2.50 |
The break-even sits between 6 kg and 8 kg. Solve it by hand: X wins once 11.00 times n plus 4.50 is below 12.40 times n, which means 1.40 times n must be bigger than 4.50. That needs n of 4 or more. For a household that only needs 2 kg, the lower shelf price is a trap. For a group buy of 8 kg, it is the right call.
Step 5: buy whole packs and count the surplus
You cannot buy 0.75 of a pack. If you need 1.5 kg and the offer is a 2 kg pack, you pay for 2 kg and 500 g goes unused. Whether that matters depends on whether you will use it.
| Option | Packs bought | You receive | Checkout | Unused surplus | Cost per 100 g you need |
|---|---|---|---|---|---|
| One 2 kg pack at 8.00 | 1 | 2 kg | 8.00 | 500 g, about 2.00 of the total | 0.53 |
| 500 g packs at 2.50 | 3 | 1.5 kg | 7.50 | None | 0.50 |
The 2 kg pack has the better unit price at 0.40 per 100 g, against 0.50 for the small packs. It is still the more expensive purchase for a 1.5 kg need, by 0.50. The surplus is worth 8.00 times 500 divided by 2,000, which is 2.00, and you only recover it if you use or store it.
This is the check that most unit-price labels cannot make for you, because the label does not know how much you need. Write your quantity down before you compare.
Step 6: round once, in the smallest coin
Money is paid in whole cents, pence or yen, and the sums above round at exactly one place: the discount. For example, 12% off 33.45 is 4.014, which is 4.01. 15% off 24.99 is 3.7485, which is 3.75.
Unit prices deserve one more digit than the money, because the difference between two offers is often under a cent. A 450 g pack at 398 yen comes to 88.4 yen per 100 g, while a 1.5 kg pack at 1,180 yen comes to 78.7. Currencies with no minor unit, such as yen, still need that decimal in the comparison even though the till never charges it. Currencies with three decimals, such as the Kuwaiti dinar, go the other way and keep all three on the total.
A two-minute checklist for any multipack
- Write down paid packs, free packs, the amount in each pack and the displayed total.
- Convert every amount to grams, millilitres or items.
- Divide the total by the quantity you receive and scale to 100 g, 100 ml or one item.
- Check the coupon minimum against the items subtotal. At the minimum exactly, it applies.
- Add delivery once.
- Enter the quantity you need, buy whole packs, and note the surplus.
- Rank by the checkout total at that quantity, not by the shelf price.
You can run all seven steps with a pencil. If you would rather not, the unit price comparison does the same sums for up to eight offers, shows each line of the arithmetic, and exports the result as a CSV or a printable sheet. It is built for shoppers; if you run a shop and want the other side of the season, read the 10-day website plan for Black Friday.
Frequently asked questions
What is the best unit for comparing multipacks?
Use 100 g for weight, 100 ml for volume and one item for counted goods. They keep the numbers readable and put offers measured in kilograms, pounds, litres or fluid ounces on the same scale after conversion.
Does a coupon that needs a minimum spend ever make a bigger basket cheaper?
Yes. When the discount is larger than the cost of the item that lifts you over the minimum, the larger basket costs less at checkout. For example, 21.50 with a 15% coupon costs 18.27, which is less than a 19.00 basket without it. It is only a saving if you would use the extra item.
How do I compare a free-delivery offer with a cheaper offer that charges for delivery?
Add the delivery to the cheaper offer once, then compare checkout totals at the quantity you need. Because delivery is a fixed cost, the offer with the higher shelf price often wins for small orders and loses for large ones, as the 2 kg example shows.
Why not just pick the lowest price per 100 g?
Because the shelf price ignores whole packs, surplus, delivery and the coupon minimum. The lowest unit price tells you which pack is the best value per gram. The lowest checkout total at your needed quantity tells you which one costs least in practice.