Irish Calculators
VAT & tax arithmetic

Add or Remove a Known Tax Percentage Calculator

Find the tax and before-or-after-tax amount from a percentage you already know. The tool applies that rate arithmetically; it does not identify the tax or determine the correct legal rate.

Runs privately in your browserMethod reviewed

Your numbers

€
%
Your figures stay on this device.

How the calculation works

This is percentage arithmetic for a tax rate you already know. It can add that percentage to a before-tax amount or recover the tax contained in a total. It does not identify a tax, determine a rate or infer a percentage from two separate prices.

Add tax: net × rate. Extract included tax: gross − gross ÷ (1 + rate).

  1. Establish the before-tax base

    In add mode the entered amount is already the base. In extract mode divide the inclusive total by one plus the rate expressed as a decimal.

  2. Recover the matching total

    Multiply that base by one plus the decimal rate. This gives a consistent before-tax and inclusive pair for the selected scenario.

  3. Identify the tax component

    Multiply the before-tax base by the percentage divided by one hundred. This identifies the tax component separately from the whole inclusive price and is mathematically equivalent to subtracting the base from that total.

Worked example: Add 5%

Sample inputs demonstrate the method. They are not a forecast or product offer.

The first example supplies €100 before tax and a 5% rate. Five percent of €100 is €5, so the inclusive amount is €105. The tax result is the €5 difference, not the entire €105 total.

Amount (€)
100
Tax percentage (%)
5
Amount entered
Before tax

Expected result

Tax amount
€5.00
Worked example: Extract 20%

Sample inputs demonstrate the method. They are not a forecast or product offer.

Amount (€)
120
Tax percentage (%)
20
Amount entered
Including tax

Expected result

Tax amount
€20.00

Understanding your result

Tax is a percentage of the before-tax base. It is therefore a smaller percentage of the inclusive total; multiplying that total directly by the stated rate would overstate the embedded tax.

The tool models one proportional charge. It does not calculate progressive income-tax bands, deductions, reliefs, transaction eligibility or a payroll net amount.

Questions you might have

What does this tax percentage calculator include?

Apply a tax percentage you specify to an amount, or extract tax already included in a gross total.

Which assumptions should I check?

The entered percentage is a calculation assumption. This does not identify which tax, relief or legal rate applies to a transaction.

Choosing the right inputs

Amount
Enter either the before-tax base or the inclusive total, according to the selected mode. The amount must be non-negative.
Tax percentage
Supply the percentage that applies to your scenario. A value of 5 means five percent, not a decimal fraction of 0.05.
Amount entered
Choose before tax to add the charge, or including tax to extract a charge already embedded in the amount.

Common mistakes to avoid

  • Do not choose add mode for a total that already contains the same tax.
  • Displayed money is rounded for readability; use the dedicated VAT tool when you need its explicit cent-balancing convention.

Assumptions to check

  • The entered percentage is a calculation assumption. This does not identify which tax, relief or legal rate applies to a transaction.

Sources and method

Read our calculation and testing approach.