Reverse Tax Formula Explained: How to Calculate Tax Backwards
When a price already includes tax, you cannot usually find the original price by subtracting the tax percentage from the final total.
Instead, you must reverse the multiplier that created the tax-inclusive price.
The basic reverse tax formula is:
Price before tax = Tax-inclusive total ÷ (1 + Tax rate ÷ 100)
After calculating the price before tax, find the included tax with:
Included tax = Tax-inclusive total - Price before tax
For example, if a final price of 120.00 includes 20% tax:
120.00 ÷ 1.20 = 100.00
The original price is 100.00, and the included tax is:
120.00 - 100.00 = 20.00
This guide explains how the reverse tax formula works, how to enter rates correctly, which formula to use for different goals, and when a simple calculation is not enough.
For the complete formula reference and related calculation resources, read the main Reverse Tax Formula guide.
What Is the Reverse Tax Formula?
The reverse tax formula separates a final tax-inclusive amount into:
Price before tax
Tax included in the total
Tax multiplier used to create the total
It works backwards from an amount that already includes sales tax, VAT, GST, HST, or another percentage-based tax.
Suppose:
Tis the tax-inclusive totalris the tax rate as a decimalPis the price before tax
The formula is:
P = T ÷ (1 + r)
The included tax is:
Tax = T - P
When the rate is written as a percentage rather than a decimal, use:
P = T ÷ (1 + Rate ÷ 100)
Why the Formula Divides
Tax is normally added by multiplication.
Suppose a product costs 100.00 before tax and the rate is 8%.
Convert the rate to a decimal:
8% = 0.08
Add 1 to create the multiplier:
1 + 0.08 = 1.08
Apply the multiplier:
100.00 × 1.08 = 108.00
The final tax-inclusive price is 108.00.
To reverse the calculation, use the opposite operation:
108.00 ÷ 1.08 = 100.00
Division reverses the multiplication that originally created the final total.
Why Subtracting the Rate Is Wrong
A common mistake is:
Tax-inclusive total × (1 - tax rate)
For a total of 108.00 at 8%, this gives:
108.00 × 0.92 = 99.36
The correct price before tax is 100.00.
The subtraction formula removes 8% of 108.00:
108.00 × 8% = 8.64
However, the actual tax was calculated from the original 100.00:
100.00 × 8% = 8.00
The incorrect formula removes 0.64 too much.
Subtracting the rate from the final total behaves like applying a discount. It does not undo the tax multiplier.
Reverse Tax Formula for Price Before Tax
Use this formula when you know:
The final amount includes tax
The applicable tax rate
One rate applies to the entire amount
The tax amount is not already known
Formula with a percentage rate:
Price before tax = Total ÷ (1 + Rate ÷ 100)
Formula with a decimal rate:
Price before tax = Total ÷ (1 + Decimal rate)
Example with 8%
Suppose:
Tax-inclusive total: 216.00
Rate: 8%
Create the multiplier:
1 + 8 ÷ 100 = 1.08
Divide:
216.00 ÷ 1.08 = 200.00
The price before tax is 200.00.
Reverse Tax Formula for Included Tax
Once the price before tax has been calculated, subtract it from the final total.
Included tax = Total - Price before tax
Using the previous example:
216.00 - 200.00 = 16.00
The result is:
Price before tax: 200.00
Included tax: 16.00
Tax-inclusive total: 216.00
You can also calculate the tax directly.
With the rate written as a decimal:
Included tax = Total × (Rate ÷ (1 + Rate))
With the rate written as a whole percentage:
Included tax = Total × (Rate ÷ (100 + Rate))
For 216.00 at 8%:
216.00 × (8 ÷ 108) = 16.00
Both approaches produce the same result.
The two-step method is often easier to understand and audit because the pre-tax price and included tax should add back to the original total.
Formula Symbols Explained
| Symbol or term | Meaning |
|---|---|
| Total | Final amount that already includes tax |
| Rate | Applicable tax percentage |
| Rate ÷ 100 | Rate converted into decimal form |
| 1 + Rate ÷ 100 | Tax multiplier |
| Price before tax | Original amount before tax was added |
| Included tax | Tax contained inside the final total |
Understanding these terms helps prevent formula and spreadsheet errors.
Percentage Rate Versus Decimal Rate
The same tax rate may be written in different formats.
| Percentage | Decimal | Multiplier |
|---|---|---|
| 5% | 0.05 | 1.05 |
| 7% | 0.07 | 1.07 |
| 8% | 0.08 | 1.08 |
| 10% | 0.10 | 1.10 |
| 13% | 0.13 | 1.13 |
| 20% | 0.20 | 1.20 |
When the rate is written as 8, divide it by 100 inside the formula:
Total ÷ (1 + 8 ÷ 100)
When the rate is already written as 0.08, do not divide it by 100 again:
Total ÷ (1 + 0.08)
Both formulas use a divisor of 1.08.
Example 1: Reverse 5% Tax
Suppose a final price of 105.00 includes 5% tax.
Find the multiplier:
1 + 5 ÷ 100 = 1.05
Find the pre-tax price:
105.00 ÷ 1.05 = 100.00
Find the tax:
105.00 - 100.00 = 5.00
Result:
Price before tax: 100.00
Included tax: 5.00
Total: 105.00
Example 2: Reverse 8% Tax
Suppose the tax-inclusive total is 270.00.
Rate:
8%
Multiplier:
1.08
Price before tax:
270.00 ÷ 1.08 = 250.00
Included tax:
270.00 - 250.00 = 20.00
Result:
Price before tax: 250.00
Included tax: 20.00
Total: 270.00
Example 3: Reverse 10% Tax
Suppose the final total is 110.00.
Divide by 1.10:
110.00 ÷ 1.10 = 100.00
Calculate included tax:
110.00 - 100.00 = 10.00
Do not calculate:
110.00 × 10% = 11.00
That would calculate 10% of the final total rather than extract the tax calculated from the original base.
Example 4: Reverse 13% Tax
Suppose an invoice total is 565.00 and includes 13% tax.
Price before tax:
565.00 ÷ 1.13 = 500.00
Included tax:
565.00 - 500.00 = 65.00
Result:
Price before tax: 500.00
Included tax: 65.00
Total: 565.00
Example 5: Reverse 20% VAT
Suppose a VAT-inclusive price is 240.00.
Price before VAT:
240.00 ÷ 1.20 = 200.00
VAT included:
240.00 - 200.00 = 40.00
Result:
Price before VAT: 200.00
VAT included: 40.00
VAT-inclusive total: 240.00
Reverse Sales Tax Formula
The reverse sales tax formula uses the same mathematical relationship:
Price before sales tax = Total after sales tax ÷ (1 + Sales tax rate ÷ 100)
Then:
Sales tax included = Total after sales tax - Price before sales tax
Example:
Final total: 108.00
Sales tax rate: 8%
Calculation:
108.00 ÷ 1.08 = 100.00
Included sales tax:
108.00 - 100.00 = 8.00
The phrase “reverse sales tax” is narrower than “reverse tax.” The general formula can also be used for simple percentage-based VAT, GST, HST, PST, and similar taxes when one rate applies to the amount.
Reverse VAT Formula
For a VAT-inclusive price:
Price excluding VAT = VAT-inclusive total ÷ (1 + VAT rate ÷ 100)
Then:
VAT included = VAT-inclusive total - Price excluding VAT
Example with 20% VAT:
180.00 ÷ 1.20 = 150.00
VAT:
180.00 - 150.00 = 30.00
Reverse GST Formula
For a GST-inclusive total:
Price before GST = GST-inclusive total ÷ (1 + GST rate ÷ 100)
Example with 5% GST:
210.00 ÷ 1.05 = 200.00
GST included:
210.00 - 200.00 = 10.00
Which Reverse Tax Formula Should You Use?
The correct formula depends on what information you know and what you need to calculate.
| What you know | What you need | Formula |
|---|---|---|
| Total and rate | Price before tax | Total ÷ (1 + Rate) |
| Total and pre-tax price | Tax amount | Total - Pre-tax price |
| Total and shown tax amount | Pre-tax price | Total - Tax amount |
| Tax amount and rate | Original price | Tax amount ÷ Rate |
| Pre-tax price and total | Implied rate | (Total - Pre-tax price) ÷ Pre-tax price |
| Pre-tax price and rate | Final total | Pre-tax price × (1 + Rate) |
| Multiple rates | Grouped base and tax | Separate each rate group |
Different reverse-tax questions require different formulas.
When Subtraction Is Correct
Subtraction is correct when the actual tax amount is known.
Suppose a receipt shows:
Final total: 108.00
Tax: 8.00
Price before tax:
108.00 - 8.00 = 100.00
You are subtracting a known monetary amount.
The mistake is subtracting 8% from the final total when the tax amount itself is unknown.
Remember:
Known rate: divide by the multiplier
Known tax amount: subtract the amount
How to Verify the Reverse Tax Formula
After calculating the pre-tax price, apply the tax forward.
Suppose:
Calculated pre-tax price: 200.00
Tax rate: 8%
Calculate tax:
200.00 × 8% = 16.00
Rebuild the total:
200.00 + 16.00 = 216.00
Or use the multiplier:
200.00 × 1.08 = 216.00
Compare the rebuilt total with the original tax-inclusive amount.
If the numbers do not match, review:
Tax rate
Percentage conversion
Starting amount
Rounding
Taxability
Exempt items
Multiple tax groups
Discounts or fees
Reverse Tax Formula in Excel
Suppose:
A2 contains the tax-inclusive total
B2 contains the tax rate as a percentage
Use this formula for the price before tax:
=A2/(1+B2)
Calculate the included tax in C2 with:
=A2-(A2/(1+B2))
Alternatively, if the pre-tax price is in C2:
=A2-C2
Example
| Cell | Value |
|---|---|
| A2 | 216.00 |
| B2 | 8% |
| C2 | =A2/(1+B2) |
| D2 | =A2-C2 |
Results:
C2: 200.00
D2: 16.00
Excel Formula When the Rate Is a Whole Number
If B2 contains 8 rather than 8%, use:
=A2/(1+B2/100)
For included tax:
=A2-(A2/(1+B2/100))
Do not use this formula when B2 already contains 8%, because Excel treats 8% as 0.08.
Common Reverse Tax Formula Mistakes
Mistake 1: Subtracting the rate
Incorrect:
Total × (1 - Rate)
Correct:
Total ÷ (1 + Rate)
Mistake 2: Multiplying the total by the rate
Incorrect for extracting included tax:
Total × Rate
Correct:
Total - (Total ÷ (1 + Rate))
Mistake 3: Entering 8 instead of 8%
Entering 8 into a formula expecting 0.08 can produce a divisor of 9 instead of 1.08.
Mistake 4: Dividing by 1 plus the percentage number
At 20%, do not divide by 21.
The correct multiplier is:
1 + 20 ÷ 100 = 1.20
Mistake 5: Using the wrong rate
A mathematically correct formula still produces the wrong real-world result when the rate is incorrect.
Mistake 6: Reversing an amount that excludes tax
Do not divide a tax-exclusive subtotal by the multiplier.
Mistake 7: Applying one rate to mixed items
Separate taxable, exempt, zero-rated, and differently taxed items before calculating.
What If the Total Contains Exempt Items?
Suppose a receipt contains:
Taxable amount including 8% tax: 108.00
Exempt item: 50.00
Final total: 158.00
Do not reverse the entire 158.00 at 8%.
Reverse only the taxable amount:
108.00 ÷ 1.08 = 100.00
Tax included:
108.00 - 100.00 = 8.00
Receipt breakdown:
| Component | Amount |
|---|---|
| Taxable price before tax | 100.00 |
| Included tax | 8.00 |
| Exempt item | 50.00 |
| Total | 158.00 |
What If Several Tax Rates Apply?
Separate the amount into rate groups.
Suppose:
108.00 includes 8%
120.00 includes 20%
First group:
108.00 ÷ 1.08 = 100.00
Tax:
8.00
Second group:
120.00 ÷ 1.20 = 100.00
Tax:
20.00
Combined result:
Total before tax: 200.00
Included tax: 28.00
Final total: 228.00
Do not select one rate and apply it to the combined amount.
What About Multiple or Stacked Taxes?
Some transactions contain more than one tax.
Taxes may be:
Added independently to the same base
Applied one after another
Combined into one displayed rate
Split between different authorities
The order matters.
If two taxes are both calculated from the same base, their rates may be added before creating the multiplier.
If the second tax is applied after the first tax, the multipliers may need to be applied separately.
Do not assume every multi-tax transaction can be reversed with one simple combined rate.
How Discounts Affect the Formula
A discount may reduce the taxable base before tax.
Suppose:
Original price: 100.00
Discount: 10.00
Taxable price after discount: 90.00
Tax rate: 8%
Tax:
90.00 × 8% = 7.20
Final total:
90.00 + 7.20 = 97.20
Reverse the final total:
97.20 ÷ 1.08 = 90.00
The reverse-tax result is the discounted price before tax, not the original list price.
Shipping, Tips, Fees, and Credits
A payment total may contain components with different treatment, including:
Shipping
Delivery
Service charges
Optional tips
Handling fees
Deposits
Store credits
Gift cards
Previous payments
Before applying the formula, determine which components:
Include tax
Use the same rate
Should be separated
Represent payment rather than taxable value
The final card payment is not always the correct amount to reverse.
Why the Result May Differ by One Cent
Receipts may calculate tax:
Per item
Per line
Per tax category
On the combined subtotal
Each stage may use different rounding.
Suppose a calculator reverses the complete receipt and rounds once, while the seller rounded each line separately.
The two results may differ by one cent.
A small difference may reflect rounding order rather than an incorrect formula.
A larger difference may indicate:
Wrong rate
Wrong starting amount
Mixed tax groups
Exempt items
Additional fees
Incorrect percentage formatting
When the Basic Formula Works Best
The standard formula works cleanly when:
The total includes tax
The rate is known
One rate applies
Every amount in the total is taxable at that rate
No unrelated credits or payments are included
Rounding differences are understood
When these conditions are not met, separate the transaction into clean groups first.
Calculate Your Own Tax-Inclusive Amount
For a clean total that includes tax at one known rate, use the free Reverse Tax Calculator.
Enter the final total and applicable rate to calculate:
Price before tax
Included tax
Tax multiplier
Calculation breakdown
For mixed-rate or partly exempt receipts, calculate each taxable group separately.
Final Takeaway
The reverse tax formula is:
Price before tax = Total ÷ (1 + Rate ÷ 100)
Then:
Included tax = Total - Price before tax
The key principle is that tax was added through multiplication, so it must be reversed through division.
Do not subtract the percentage from the final total, and do not multiply the tax-inclusive amount directly by the listed rate to find the included tax.
Before calculating, confirm that the total, rate, and taxable base all describe the same transaction.
For a complete formula hub with additional explanations and related calculations, visit the main Reverse Tax Formula resource.
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