How this instrument works
In a VAT jurisdiction the sticker price is legally the tax-inclusive one — the customer pays £118 and the retailer never sees that whole figure as revenue, because a fifth of it is government money passing through the till on its way to the tax authority. Treat £118 as the sale and compare it straight against a £60 cost, and the arithmetic answers a question nobody asked: it flatters the margin by counting tax the seller never keeps.
The instrument runs two steps in order because the order matters. First it divides the inclusive price by one plus the tax rate to recover what the sale earns before that levy — the money the business actually retains before its own costs. Only then does it subtract cost and divide by that same recovered figure to get margin. Reverse the steps, or skip the division, and every downstream number is wrong by exactly the rate baked into the sticker.
This matters most to anyone who prices where sales tax is quoted this way and thinks in margin: an ecommerce owner setting a round tax-inclusive price and needing to know what it really nets, a shop manager checking whether a promotional price still clears a minimum margin once the levy is peeled off, or a bookkeeper reconciling till receipts against a cost sheet. A US sales-tax retailer never hits this trap, because that tax is quoted and charged on top of the shelf price rather than folded inside it.
- Enter Cost, $ — what the item cost the business to buy or make, excluding any VAT the business itself reclaims on purchases.
- Enter VAT-inclusive selling price, $ — the tax-inclusive figure printed on the shelf or the invoice.
- Set VAT rate, % to the rate that actually applies to this item — standard, reduced, or zero, depending on the goods.
- Read Price excluding VAT for the money the sale really contributes, then Margin, % for the true figure beneath it.
Worked example — a £118 price, a £60 cost
An item costs £60 and sells for £118 with 20% VAT already included in that sticker figure. Dividing first: £118 ÷ (1 + 20 ÷ 100) = £118 ÷ 1.20 = £98.33 — the amount the sale actually generates before the tax authority takes its cut. The margin follows from that number alone: (£98.33 − £60) ÷ £98.33 × 100 ≈ 38.98%.
Skip the division and a seller working straight from the sticker price gets (£118 − £60) ÷ £118 × 100 ≈ 49.15% — a full ten points higher, and a mistake this instrument's second step catches. That gap is not rounding noise; it is the 20% rate still sitting inside the numerator and denominator, making a genuinely thinner margin look comfortably fat.
Questions
Why can't I just use (price − cost) ÷ price when the price includes tax?
Because a tax-inclusive price is not revenue — a fixed slice of it is money the business collects and remits, never keeps. Dividing straight from that figure inflates margin by the rate baked in; on a 20% VAT sale the error runs about ten percentage points, exactly the size shown in the worked example above.
What if I already know the ex-VAT price and don't want to enter a tax-inclusive figure?
Set VAT rate, % to 0 and type that figure into VAT-inclusive selling price, $. With a zero rate the two are identical by definition, so Price excluding VAT returns the same number back and the margin is computed correctly with nothing stripped out.
Does the rate I enter need to match the item exactly?
Yes. Standard VAT rates in the European Union run roughly 17% to 27% by country, and most jurisdictions also carry reduced or zero rates for categories like food, books, or children's clothing. Entering the standard rate for a reduced-rate item overstates what gets stripped out and understates the true margin.
Is VAT a cost that should reduce my margin?
No. It is a pass-through the business collects on the tax authority's behalf and later remits, not money it spends or keeps, so it does not belong in either the revenue or the cost side of a margin calculation. That is precisely why the first step here removes it before the margin arithmetic ever runs.
Why does the margin on paper never match the markup I set at cost times 1.4?
A markup applied to cost and a margin measured against price answer different questions even before tax enters the picture; a 40% markup on £60 produces an £84 pre-tax price, which is only a 28.6% margin, not 40%. Add tax-inclusive pricing on top and a spreadsheet built around markup alone will drift further from the margin the business actually reports.
Can this handle a reverse charge or zero-rated sale?
Enter 0 for VAT rate, % — that models a zero-rated or reverse-charge sale where none is added to the price the customer pays, and the two price figures collapse to the same number. It does not track the separate reclaim or reporting obligations those categories carry; consult a tax adviser for how to file them.
References
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.