Reverse sales tax calculator, Ontario

Why you cannot just subtract 13%

You have a receipt for $100.00 in Ontario and you need the price before tax. The instinct is to take 13% off the total. That gives $87.00, and it is wrong by $1.50.

The reason is that the tax was never calculated on $100.00. It was calculated on the smaller number — the pre-tax price — and then added. So to undo it you divide by 1.13, not subtract a percentage. The right answer is $88.50 before tax and $11.50 of tax.

The error is proportional, so it grows with the receipt: on $10,000 it is $149.56. It is the single most common arithmetic mistake in expense claims, and it always goes the same way — understating the pre-tax cost, and therefore the tax you could claim back.

When you actually need this

Expense claims and bookkeeping. A business reclaims the GST/HST it paid as an input tax credit, and to do that it needs the tax split out. A till receipt that only shows a total forces the calculation.

Invoicing backwards from a round number. Quoting "$1,000 all in" means the price before tax is $884.96, not $1,000. Get that wrong on a quote and the difference comes out of your margin.

Checking a receipt. Dividing the total and comparing the tax line is the fastest way to spot a till charging the wrong rate — which happens most often in the months after a rate change.

The arithmetic, in full

For Ontario, with a combined rate of 13%:

Price before tax = total ÷ 1.13

HST = price before tax × 0.13

Worked on $100.00: $100.00 ÷ 1.13 = $88.50, of which $11.50 tax.

There is only one tax to reverse here, which makes Ontario the easy case.

Why anyone does this: the receipt that is allowed not to tell you

Almost nobody reverses a receipt out of curiosity. They do it because a registered business recovers the GST/HST it paid through an input tax credit, and to claim one it needs the tax as a separate figure.

Which runs into the thing that makes this page necessary. Below $100, the receipt is not required to show the tax at all. The Canada Revenue Agency asks for three things on a purchase under that amount: the supplier’s name or trading name, the date of the invoice, the total amount paid or payable. That is the whole list. So the corner-shop receipt in your wallet is not badly made — it is complying, and the tax has to be derived by dividing.

From $100 the requirements step up: you also need the supplier’s GST/HST registration number and an indication of the total amount of GST/HST charged. And from $500, on top of that, the buyer’s name or trading name, a brief description of the property or services, the terms of payment. So the receipts you most need to reverse are precisely the small ones, where the law asks least.

In Ontario the split is simpler, and in your favour: there is no separate provincial sales tax, so the whole $11.50 on a $100.00 receipt is HST and the whole of it is recoverable. That is not true everywhere — in British Columbia, Saskatchewan and Manitoba part of the tax on the same receipt would never come back, because provincial sales tax is not an input tax credit.

Read off CRA — Input tax credits, and the documentary requirements that apply by amount of the purchase, on 2026-09-21.

The cent that will not reconcile

If you reverse receipts one at a time this never comes up. If you reverse a hundred of them into a ledger, it comes up on the first afternoon.

The total you start from was already rounded to the cent when the till printed it. Dividing it gives a base with more decimal places than money has, and rounding that to the cent leaves a figure which, multiplied back up, does not always return the total you started with.

In Ontario, at 13%: a total of $10.13 reverses to $8.96 before tax. Put 13% back on $8.96 and you get $10.12 — a cent adrift from where you began. Neither figure is wrong; the cent was lost when the till rounded, and no arithmetic gets it back.

Which is why bookkeeping software posts the tax as the difference between the total and the rounded base, rather than calculating it from the base directly. The total is the figure that actually changed hands and it is the one that has to be right; the base absorbs the rounding. This calculator shows both, and on a receipt where they disagree by a cent, the total wins.

It also means a reversed figure should never be used to check whether a shop charged the correct rate on a small purchase — a cent of difference proves nothing. On a $2,000 invoice it would; on a $10.13 receipt it is just rounding.

The receipt where dividing gives the wrong answer

Everything above assumes one thing, and it is worth saying out loud: that every line on the receipt was taxed at the same rate. On a great many Canadian receipts it was not, and then no single division can recover the split.

Zero-rated goods are the common case. Basic groceries, prescription drugs and most medical devices carry GST/HST at zero per cent — not exempt, formally, but rated at zero, which is the same thing at the till and a different thing on the seller's return. So a supermarket bill is two receipts printed as one.

Put numbers on it. A $105.20 Ontario shop made of $60.00 of basic groceries and $40.00 of taxable goods carries $5.20 of tax — all of it on the $40.00. Divide the $105.20 total by 1.13 and you get $93.10 before tax and $12.10 of tax: $6.90 more tax than was actually charged, invented out of groceries that were never taxed.

The tell is on the receipt itself. A till that handles mixed baskets prints the taxable subtotal separately, usually flagged line by line with a letter. Where that subtotal exists, use it and skip the division entirely — you are not deriving anything, you are reading it.

And the case that looks like a till error and is not. Ontario refunds its provincial share at the point of sale on books, children's clothing and footwear, children's diapers, children's car seats, qualifying newspapers, qualifying food and beverages, so those lines come through at the federal 5% while everything else on the same receipt carries the full 13%. Three rates on one piece of paper, and the total tells you about none of them.

Either way this is the point at which a calculator stops being the right tool and the taxable subtotal printed on the paper becomes the only honest source.

And when the paper does not carry one — an old till roll, a handwritten receipt — the answer is not to divide anyway. It is to add up the taxable lines yourself and reverse that subtotal, which is the same division applied to the right number. The arithmetic on this page was never the difficult part; knowing which figure to feed it is.

Questions

How do I remove 13% tax from a total in Ontario?

Divide by 1.13. A $100.00 total is $88.50 before tax and $11.50 of tax. Subtracting 13% from the total instead gives $87.00, which is wrong by $1.50.

Why is subtracting the percentage wrong?

Because the tax was calculated on the pre-tax price, not on the total. Taking 13% off the total removes 13% of a bigger number than the one the tax was worked out on, so you take away too much. The error is proportional and grows with the receipt.

Does this work for a receipt with exempt items on it?

No, and that is the real limitation. If part of the bill was zero-rated — groceries, prescriptions — the tax was charged on only part of the total, and dividing the whole thing gives the wrong split. This works for a receipt where everything was taxable.

How do I back out HST from a total in Ontario?

Divide by 1.13. A $100.00 total is $88.50 before tax and $11.50 of tax. Backing out, removing, reversing and working out the pre-tax price are all the same division — what they are not is subtracting 13% from the total.

Why does my receipt in Ontario not show the tax separately?

Under $100 it is not required to. The CRA asks only for the supplier's name, the date and the total on a purchase that size, so the tax has to be derived by dividing. From $100 the receipt must also carry the supplier's GST/HST registration number and the tax charged.