Discount Calculator Formula
Learn how to calculate sale price, amount saved, effective discount, and final price after tax.
This page explains the math behind a discount calculator. It shows how to convert a percentage or fixed discount into savings, subtract that savings from the original price, and optionally add sales tax to estimate the final amount paid.
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Final Price After Discount and Tax
Where:
First find the discount amount, subtract it from the original price to get the sale price, then calculate tax on the discounted price and add it back to get the final price.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| originalPrice - Original price | The starting price before any discount is applied. | currency |
| discountPercent - Discount percentage | The percentage taken off the original price when using a percent discount. | percent |
| discountAmount - Discount amount | The fixed amount taken off the original price when using a fixed discount. | currency |
| amountSaved - Amount saved | The actual discount applied after any cap is considered. | currency |
| salesTaxRate - Sales tax rate | The tax rate applied to the discounted price. | percent |
| salePrice - Sale price | The price after discount and before tax. | currency |
| taxAmount - Tax amount | The estimated tax calculated on the discounted price. | currency |
| finalPrice - Final price | The total estimated price after discount and tax. | currency |
Step-by-Step Calculation
Calculate the discount from a percentage
If the discount is percentage-based, multiply the original price by the discount rate.
discountFromPercent = originalPrice * (discountPercent / 100)
Calculate the discount from a fixed amount
If the discount is a fixed amount, cap it at the original price so the price cannot go below zero.
discountFromAmount = min(originalPrice, discountAmount)
Set the actual amount saved
Use the result that matches the selected discount type.
amountSaved = discountFromPercent or discountFromAmount
Find the sale price before tax
Subtract the savings from the original price.
salePrice = originalPrice - amountSaved
Calculate the effective discount rate
This shows the actual savings as a percentage of the original price.
effectiveDiscountPercent = (amountSaved / originalPrice) * 100
Calculate estimated tax
Apply tax to the discounted price, not the original price.
taxAmount = salePrice * (salesTaxRate / 100)
Calculate the final price
Add estimated tax to the discounted price to get the final total.
finalPrice = salePrice + taxAmount
Example: 20% off a $100 item with 8% sales tax
Discount amount
100 × (20 / 100)
$20.00
Sale price before tax
100 - 20
$80.00
Effective discount
(20 / 100) × 100
20%
Tax amount
80 × (8 / 100)
$6.40
Final price after tax
80 + 6.40
$86.40
Final Result
The item costs $80.00 before tax, you save $20.00, and the estimated final price after tax is $86.40.
Assumptions
- ✓The discount is applied once to the original price.
- ✓Sales tax is calculated after the discount is applied.
- ✓A fixed discount cannot exceed the original price.
- ✓The calculation does not include shipping, handling, or extra fees.
Limitations
- !Actual checkout totals may differ if a store taxes items differently.
- !The calculator does not model stacked coupons or multiple discount stages.
- !It does not include store-specific rules such as minimum spend requirements.
- !If the original price is zero, the effective discount percentage is not meaningful.
Common Mistakes to Avoid
Entering a percentage discount as a whole currency amount.
Forgetting that tax is applied to the discounted price in this calculator.
Assuming a fixed discount can make the price negative.
Comparing two deals without checking whether one uses pre-tax or after-tax totals.
Using the sale price as the original price for a second separate calculation by mistake.
Related Formulas
Frequently Asked Questions
How do you calculate a discount percentage from the original price?
Divide the amount saved by the original price, then multiply by 100.
How do you calculate sale price after a discount?
Subtract the discount amount from the original price.
Is tax calculated before or after the discount in this formula?
This calculator assumes tax is calculated after the discount is applied.
What happens if the fixed discount is larger than the original price?
The discount is capped at the original price so the sale price does not go below zero.
Can the effective discount differ from the entered discount?
Yes, especially when using a fixed amount discount, because the effective rate depends on the original price.
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