Buy X Get Y Calculator

Set the threshold above the normal basket.

Set the threshold above the normal basket. Setting the buy quantity at or below the normal basket means giving away units people were already buying.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Currency

Price

Contribution margin

Buy quantity

Get free

Units in a normal basket

Contribution per deal

$44.08

25% effective discount

Units in the deal4

Effective discount25%

Contribution from a normal basket$45.94

Extra units the deal requires2.2

Setting the threshold at or below the normal basket means giving away units people were already buying. Raise the buy quantity above 1.8 and the deal starts creating volume rather than discounting it.

How the Buy X Get Y Calculator works

Setting the buy quantity at or below the normal basket means giving away units people were already buying. Setting it just above forces the basket up, which is the only mechanism by which these offers create anything.

Also known as: buy 2 get 1 free calculator · multibuy discount value · quantity promotion calculator

The general form

Buy X get Y covers everything from buy two get one free to buy three get 20% off the fourth. The effective discount is the value given away divided by the value of the whole bundle.

Buy three get one free is a 25% discount across four units. Buy two get one half price is a 16.7% discount across three. Each is smaller than it sounds, which is the format's commercial advantage.

The higher the X, the smaller the effective discount and the more units move. Buy five get one free is a 16.7% discount that shifts six units, which for stock clearance is considerably more efficient than a 16.7% straight discount that shifts an unpredictable number.

Choosing the threshold

The X should sit above the typical purchase quantity, or the promotion is discounting behaviour that would have happened anyway.

If most customers buy two, then buy two get one free discounts every existing customer and changes nothing. Buy three get one free requires a step up and only discounts customers who take it.

The distribution of purchase quantities is the input, and most platforms will report it. Setting X at the point where a meaningful minority currently buy, so the offer is achievable but not automatic, is where the format earns its cost.

Cannibalisation and forward buying

Multi-buy offers on storable goods pull future purchases into the promotion period. The customer who would have bought one unit a month for four months buys four now at a discount, and the four subsequent months are dead.

That is fine if the objective is share or cash, and it is a loss if the objective was incremental volume. The measurement that distinguishes them is what happens in the weeks after the promotion, and it is routinely not looked at.

For perishables and consumables with a natural consumption rate, the forward buy is limited by how much the customer can use. For storable non-perishables it is limited only by their willingness to hold stock, which for a good enough offer is considerable.

The implementation details that leak

Which item is free when the basket contains different prices is the first question and the one that most often costs money. The cheapest item free is the standard and it has to be enforced.

Whether the offer applies repeatedly is the second. Buy three get one free on a basket of twelve should give four free rather than one, and the terms should say which. Customers who expect the first and get the second complain, reasonably.

Interaction with other discounts is the third and the largest source of unintended cost. A multi-buy applied on top of a sale price on top of a customer voucher can produce an order that ships at a loss, and the combination usually appears somewhere in a large enough campaign.

Measuring whether it worked

The headline measure is units moved, and it is misleading on its own. The right measure is incremental contribution: the contribution during the promotion, minus what would have happened without it.

Estimating the counterfactual is the difficult part. A prior period comparison is confounded by seasonality; a holdout group is cleaner and requires the ability to exclude a segment from the offer.

The measure most sellers actually use, revenue during the promotion against the same period last year, is confounded by everything and reliably concludes that the promotion worked. Which is why promotions tend to become permanent: the measurement never says stop.

Where to go next

The Buy X Get Y question rarely arrives on its own. These are the ones that usually come with it:

BOGO Discount Calculator — Contribution per deal against a single sale.
Bulk Discount Ladder Calculator — Fulfilment savings offset part of the discount.
Free Gift Threshold Calculator — High perceived value, low cost.
Etsy Fee Calculator — Every Etsy fee on one sale, itemised.

Not financial advice. Marketplace fees change, and they vary by country, plan and seller status. Every rate here is an editable default, not a quoted price, check the platform's current fee schedule before you price a product against it. This is not tax or business advice.

Frequently asked questions

Where should the buy quantity sit?

Just above the normal basket size, so reaching it requires adding something. At or below it, the offer is a discount on existing behaviour.

What is the effective discount?

Free units divided by total units. Buy three get one free is 25%; buy two get one free is 33%.

Does it work for expensive products?

Rarely, the basket increase required is too large. These offers work best where the unit price is low enough that adding one more is an easy decision.