Net, gross, and the tax amount: three numbers, one identity
Almost every tax calculation is really about three figures that must always agree. The net price is the amount before tax. The tax amount is what gets added. The gross price is the total the customer pays. The relationship never changes: net plus tax equals gross.
VAT (value added tax), GST (goods and services tax), HST, and consumption taxes differ in law and paperwork, but the price arithmetic is identical. That is why the VAT / GST Calculator uses one engine and simply swaps the label. Sales tax in the United States behaves the same way on a single receipt line — the mechanics below apply to it too.
Adding tax to a net price
When you know the price before tax, you multiply. The formula is Gross = Net × (1 + Rate ÷ 100), and the tax amount is simply Gross − Net.
The reason you multiply by (1 + rate) rather than adding the rate separately is that it does both steps at once: it keeps the original 100% of the net and layers the tax percentage on top.
Worked example
Removing tax from a tax-inclusive price
This is where most mistakes happen. If a price already includes 20% tax, you cannot just take 20% off it. The tax was charged on the net price, not on the gross, so subtracting the rate removes too much.
The correct move is to divide. Net = Gross ÷ (1 + Rate ÷ 100), then the tax is the difference. The VAT / GST Calculator does this division for you when you switch to its remove mode, which is the safe default when a shelf price or receipt is tax-inclusive.
Worked example
Why the tax is a smaller share of the gross than the rate
A 20% rate does not mean tax is 20% of the total you pay. The rate applies to the net, so the tax is 20% of a smaller number. As a share of the gross it works out lower.
From the add-tax example, tax of 40.00 on a gross of 240.00 is 40 ÷ 240 = 16.67% of the gross. This is not a rounding quirk — it is why a receipt showing tax as roughly a sixth of the total is consistent with a 20% VAT rate.
Finding the rate from two prices
Sometimes you have the before and after prices but not the rate — for example, comparing a quoted net price with a final invoice. You can recover the implied rate directly.
The calculator's reverse rate finder uses Rate = (Gross − Net) ÷ Net × 100. If the answer does not match a standard rate for the region you picked, it flags that the two prices might include something other than tax, such as shipping or a service charge.
Worked example
Invoices with mixed rates and discounts
Real invoices rarely use a single rate. A grocery order might charge the standard rate on some items and a reduced or zero rate on others. The rule is to calculate tax line by line and then total — never apply one blended rate to the whole invoice.
The calculator's invoice mode does exactly this, and it applies any discount to a line before the tax math, because tax is charged on the discounted price you actually pay. It also lets you round tax per line or at the invoice total, since both conventions are used in practice and can differ by a cent or two.
Worked example
Cross-border sales: exports, imports, and reverse charge
Domestic tax rules get more complicated the moment a sale crosses a border. Exports are typically zero-rated in VAT/GST systems — the seller charges 0% tax on the sale (rather than being exempt, which is a different legal category), and can usually still reclaim the tax paid on their own business inputs. Imports usually work the other way: goods entering a country typically have import VAT/GST charged at the border, often collected by customs alongside any duty, regardless of whether tax was charged in the country of origin.
Cross-border business-to-business services often use a "reverse charge" mechanism instead of the seller charging tax: the buyer self-assesses the tax on their own return, as if they'd both charged and paid it to themselves, and the seller issues an invoice with no tax added. This shifts the compliance burden to the buyer and avoids double taxation or under-taxation across jurisdictions. None of this changes the core net/gross/tax arithmetic in this guide — it changes which rate applies (often 0%) and who is responsible for remitting it, which is exactly the kind of detail a price calculator can't determine on its own.
Why some sellers charge no tax at all
Many VAT/GST systems (and US state sales tax) only require a business to register and start charging tax once its sales cross a registration threshold — a revenue figure that varies by country and, in the US, by state. A small seller below that threshold may legitimately charge no tax at all, which can look like an error on an invoice but is simply a rule based on scale, not a discount or exemption on the product itself.
This matters for buyers comparing prices across sellers: a lower total from an unregistered small seller isn't necessarily a better deal on the underlying goods — it may just reflect a tax obligation that seller doesn't yet have. It also means the same seller's invoices can change from tax-free to tax-inclusive partway through a year, once they cross the threshold — which is a legitimate reason a price recalculation catches you by surprise.
Common mistakes
- Subtracting the rate to remove tax (Gross − 20%) instead of dividing by 1.20 — this always understates the net price and overstates how much tax you strip out.
- Treating the rate as the share of the gross. A 20% rate is only about 16.67% of the tax-inclusive total, because the rate applies to the net, not the gross.
- Applying one average rate to a mixed-rate invoice total. Tax must be worked out per line; a blended effective rate is a summary figure, not an input.
- Adding tax before a discount. Discounts reduce the taxable amount, so apply them to the net (or gross) first, then calculate tax on the reduced figure.
- Assuming a preset rate is current. Rates and reduced-rate categories change often, so the seeded rates are reference points to verify, not authoritative values.
- Confusing per-line and invoice-total rounding. Both are legitimate and can differ by a cent or two, which matters when a filing total must reconcile exactly.
- Assuming a tax-free invoice from a small or overseas seller is always a discount. It may simply reflect that the seller hasn't crossed a registration threshold, is zero-rating an export, or the transaction falls under a reverse-charge rule — not that the product itself is cheaper.