Most "optimal threshold" advice is a guessed multiplier of AOV. This checks your actual numbers instead: how much extra a customer needs to spend to cross your threshold, versus how much extra spend it takes, at your margin, to cover the shipping you're giving away.
Free shipping threshold calculator
That common advice might land somewhere reasonable, but it doesn't reference your margin or your actual shipping cost, so it can't tell you whether a given threshold pays for itself. This calculator does two things instead: works out the gap a customer has to close to qualify (threshold − AOV), and separately works out how much extra spend, at your margin, is needed to generate enough additional profit to cover the shipping cost you're giving away.
The break-even math: giving away $7.50 of shipping at a 55% margin needs $7.50 ÷ 0.55 ≈ $13.64 of extra product revenue to generate the same $7.50 in additional gross profit. If your threshold sits $17 above current AOV, the gap comfortably covers that — assuming the extra items in a bumped-up order carry roughly your average margin, which is usually a reasonable starting assumption unless the bump is mostly coming from a specific low-margin add-on.
What this doesn't model: not every customer below the threshold decides to add more and cross it — some just accept paying for shipping, and some abandon the cart instead. That's a conversion-rate question specific to your store, not something a formula can answer from AOV and margin alone. Treat this as "does the math work if they do cross it," not a promise about how many will.
The reason that second half matters so much: in Baymard Institute's checkout survey, "Extra costs too high (shipping, tax, fees)" is the top reason US shoppers abandon a checkout they'd otherwise complete, cited by 40% of them, against a documented average cart abandonment rate of 70.22% across 50 studies. Shipping cost shown late is the single most expensive line on most checkouts — which is exactly why the "just make it free" answer is so tempting, and why it has to be paid for somewhere.
Verbatim from public ecommerce threads. Between them they cover the three outcomes this calculator can't predict for you: charging kills conversion, a threshold lifts AOV, or the gap to the threshold stalls the cart.
I sell products in the 30 to 40 dollar range. Actual shipping costs me anywhere from 6 to 9 dollars depending on the zone… I tried charging a flat 5 dollar shipping fee last week to test it out, and my conversion rate tanked by almost 40 percent.
I set a free shipping threshold at $65. Below that, flat $5.99 shipping. Two things happened: 1. AOV went from $52 to $61 in the first month (people adding items to hit the threshold) 2. The orders that still paid shipping didn't drop off — conversion stayed within 2%
I see a lot of people who probably just want one print ($29-$39) but don't even add to cart because the gap to $50 feels too big. Even worse, I have carts sitting at like $48 or $49.80.
Each is one store's experience, self-reported, not a benchmark — and note how different the products are ($30–40 consumables, a $52 AOV, single $29–39 prints). What they share is the shape of the decision: the first merchant's 40% conversion drop and the third merchant's stalled $48 carts are the two failure modes, and the second merchant's $13 AOV lift is what the calculator above is checking you'd need. Run your own gap and break-even numbers, then test on a slice of traffic before you move the threshold sitewide.