What is this calculator?
This calculator adds one or more tax percentages to a base amount, or works backwards to extract the tax already included in a gross amount. It handles split rates (for example two components of 9% each) by treating them as separate components of one combined rate.
Use it when:
- You have a pre-tax price and need the final amount payable.
- You have a tax-inclusive total and need to know the base value and the tax within it.
- A tax is levied as two or more separate components that must be shown individually on an invoice.
Intended for: Anyone preparing or checking an invoice, and buyers reconciling a quoted price against a tax-inclusive total.
How is this calculated?
- The rates you enter are summed into one combined rate — several components at 9% and 9% behave exactly like a single 18%, but each is reported separately so it can be shown on an invoice.
- In "add" mode, the base amount is multiplied by the combined rate to give the tax, which is added to the base to give the gross.
- In "extract" mode, the gross amount already contains the tax, so the base is recovered by dividing the gross by (1 + combined rate) — not by subtracting the rate, which is the most common mistake in tax-inclusive arithmetic.
- Each component's share is then apportioned from the base in proportion to its own rate.
Formulas
Adding tax
Gross = Base × (1 + Σrates / 100)
Each rate is applied to the same base, so the components simply add up.
Extracting included tax
Base = Gross / (1 + Σrates / 100)
Reverses the calculation above. Subtracting 18% from a tax-inclusive total does not recover the base and will understate it.
Example
A tax-inclusive total of 1,180 at a combined rate of 18%, split as 9% + 9%.
- The base is 1,180 ÷ 1.18 = 1,000.
- Total tax is 1,180 − 1,000 = 180.
- Each 9% component accounts for half of that: 90 each.
Result: Base amount 1,000, total tax 180 (90 + 90), gross amount 1,180.
Frequently asked questions
Why can I not just subtract 18% from a tax-inclusive price?
Because the 18% was calculated on the base, not on the gross. Subtracting 18% of the gross removes too much. Dividing by 1.18 is the correct reversal.
Why enter two rates of 9% rather than one of 18%?
Some tax regimes split a single levy into components that must be itemised separately on the invoice. Entering them separately gives you the per-component amounts while the total is unchanged.
Does this know my country's tax rates?
No, and deliberately so. The calculator is rate-neutral: you supply the rates that apply to your jurisdiction and product category, which is the only way it can stay correct as rates change.
Assumptions
What this calculator takes as given:
- All rates you enter apply to the same base amount (they are additive components, not compounded on one another).
- The rates supplied are correct for your jurisdiction and product category — the calculator holds no rate tables.
Limitations
What this calculator cannot know or does not model:
- It does not know which rate applies to which goods or services in any country.
- Cess, surcharges, reverse-charge mechanisms, and input-tax credit are not modelled.
- Rounding rules mandated by specific tax authorities for invoice values are not applied.
References
- Value-added tax — how tax-inclusive and tax-exclusive pricing differ — OECD, International VAT/GST Guidelines. Background on the general mechanics this calculator implements; consult your own tax authority for applicable rates.