August flyers are full of "25% off, plus an extra 10%!" — and most shoppers do that math wrong, then get surprised again at the register when tax lands on top. Here's the arithmetic that turns a pile of school-supply flyers into a real number you can budget for.
Stacked Discounts Don't Add
25% off plus an extra 10% off is not 35% off. Discounts apply one after another, each on the already-reduced price:
A $180 backpack-and-supplies haul at 25% off, then an extra 10% at checkout: 180 × 0.75 × 0.90 = $121.50 — a 32.5% total discount, not 35%. Small difference on one basket; real money across a season of sales.
Discount CalculatorStack sale discounts correctly, including percent-off chains
Then Comes Sales Tax
The register adds your province's rate to that discounted price. That $121.50 basket becomes $127.58 in Alberta (5% GST), $136.08 in BC (12%), $137.30 in Ontario (13% HST), and $139.73 in PEI (15%). Two provinces can differ by more than ten dollars on the same cart — worth knowing if you shop near a border.
One helpful wrinkle: most provinces with PST exempt children's clothing and footwear, and some exempt school supplies in certain cases — so the sticker math can come out better than the headline rate suggests. GST/HST still applies to most non-grocery items.
Sales Tax CalculatorYour province's exact GST/HST/PST on any price
A Budget That Survives September
- Work backwards from a total. Pick the number first ("$400 for both kids"), then divide it across categories — clothes, supplies, tech, activity fees — instead of adding up wishes and hoping.
- Price in after-tax dollars. A $400 budget in Ontario is really about $354 of sticker prices. Budgeting pre-tax is the classic way to run 13% over.
- Sequence the sales. Supplies bottom out in late August; tech deals cluster around Labour Day; clothes drop again in late September once the rush ends. Anything that can wait, should.
- Track the running total. A note on your phone with a single running sum beats every app you'll abandon by Tuesday.
The Five-Minute Version
Multiply each planned purchase by the discount factors, multiply the subtotal by 1 plus your provincial rate, and compare against the budget you set. If it's over, cut from the "wants" column — the math is impartial so you don't have to be.