We are the MonoTape team. "30% off, plus an extra 10% at the register." When that sign catches your eye, very few people can say on the spot what it actually costs. We used to get it wrong all the time. Discounts trip people up not because they're bad at arithmetic, but because stacked discounts need the in-between price — and that price stays in neither your head nor your calculator. In this article we show how to land the percent-off price every time.
Why "stacked" discounts fool people
The common mistake is "30% off plus 10% off equals 40% off." It isn't. You take 30% off first, then take another 10% off that lower price, which works out to about 37% off overall. On a $50 item, 40% off would be $30, but the right answer is $50 → $35 → $31.50. That's a $1.50 difference.
A standard calculator can do the arithmetic. But after you get 50 x 0.7 = 35, the $35 in-between price tends to vanish from the screen before you multiply by 0.9, and you lose track of what you're multiplying by what. Want to try different rates to compare? Start over from scratch.
Keep base, discounted, and taxed on separate lines
In MonoTape you name each stage and stack them up. The stacked discount and the conversion to a tax-included price all stay visible, one line at a time.
Because each stage has a name, you can trace where you took which percent and what it costs now. When someone asks "so how much did it actually drop?" you point to the base and the final price and the story explains itself. No more confusion from a $35 that disappeared mid-calculation.
Change one rate, recalculate to the final price
What you most want to know about a discount is usually "what if it were X percent?" In MonoTape you just rewrite the discount line, and every stage below it recalculates. What's the tax-included price if 30% became 40%? How much does a coupon change things? Tweak one rate and compare. Trying it on a different item is just editing the base-price line.
With discounts, the path of how the price dropped matters more than a single final figure. Name each stage and keep it, and move the rate in one place. That way you don't miss on stacked sales, and you can judge the deal on the spot.
MonoTape
A calculator notepad where you label your math and one edit recalculates everything. Discounts and taxes, estimates, split bills — whatever you calculate stays on the tape. Free to use.
Learn more about MonoTapeFrequently asked questions
Is 30% off plus 10% off the same as 40% off?
No. You take 10% off the price that's already 30% off, which is about 37% off. A $50 item comes to $31.50. Stacked discounts are "multiply in order," not "add together."
How do I calculate "X% off"?
Multiply by the share that remains. 30% off is x 0.7, 10% off is x 0.9. In MonoTape you keep each stage on its own line, so you can trace where you took which percent.
Can I change the rate to compare?
Yes. Rewrite the discount-rate line and the discounted and tax-included lines that reference it recalculate automatically. Check "what if it were 40%?" in an instant.
Is it free?
Yes. Labeled calculations, references, auto-recalculation, and tape-style history — every feature mentioned here is free. It works on both iPhone and Android.