Price changes: how to put a rise or fall in percent
Price change in percent compares a new price to an old one. Subtract the old price from the new, divide by the old, and multiply by 100. The old price is always the divisor, even when the new price feels more relevant.
TL;DR
- Percentage change measures the relative difference between a starting value and a final value
- Formula: Percentage Change % = ((New Price - Old Price) ÷ Old Price) × 100
- Example 1: Measuring inflation on electricity bills
How it works
Percentage change measures the relative difference between a starting value and a final value. It shows how fast and in which direction a price moved, so you can compare a 30 euro increase on a 50 euro item with the same increase on a 500 euro item.
The math
To calculate the percentage price change, subtract the original price from the new price to find the absolute price difference. Divide this difference by the original price, and multiply by 100 to find the percentage change.
Subtracting the old price from the new price reveals the change direction (positive for increases, negative for decreases). Dividing this difference by the historical starting price and multiplying by 100 formats it as a percentage.
Examples
Example 1: Measuring inflation on electricity bills
- Identify historical monthly bill: $50.00
- Identify new monthly bill: $65.00
- Calculate absolute change: $65.00 - $50.00 = $15.00 increase
- Divide change by starting price: $15.00 ÷ $50.00 = 0.30
- Multiply by 100 for percentage: 0.30 × 100 = 30% inflation
Example 2: Analyzing corporate inventory price reductions
- Identify original inventory cost: $120.00
- Identify markdown price: $90.00
- Calculate absolute change: $90.00 - $120.00 = -$30.00 drop
- Divide change by starting price: -$30.00 ÷ $120.00 = -0.25
- Multiply by 100 for percentage: -0.25 × 100 = -25% decrease
When to use it
✦ Investment Portfolio Performance Tracking
Stock investors track percentage price changes daily to analyze stock performance, monitor asset growth, and manage investment risk.
✦ Cost of Living and Inflation Indexing
Families use price change calculations to track inflation on grocery items, gasoline, and utilities, helping them adjust their household budgets.
✦ Corporate Profit Margin Reviews
Purchasing managers analyze raw material price variations over time to adjust wholesale purchase schedules and maintain healthy profit margins.
⚠ Common mistakes
- Dividing by the New Price: The most common math error is dividing the price difference by the newer price instead of the historical starting price. E.g. dividing $15 by $65 instead of $50 will result in incorrect percentages.
- Confusing Relative Change with Absolute Values: Assuming that equal percentage changes indicate equal absolute dollar shifts, which is incorrect when starting values differ.
FAQ (7)
What is the difference between percentage change and percentage points?
Percentage change measures relative growth compared to a starting value. Percentage points simply measure the arithmetic difference between two percentages (e.g. rising from 5% to 8% is a 3 percentage point rise, but a 60% relative increase).
Can a price drop exceed 100%?
No, a price drop of 100% means the final item is completely free ($0.00). Unless you are being paid to take an item, retail price reductions cannot exceed 100%.
Why is the starting value always the divisor?
Because percentage change measures how much a value grew or shrunk relative to its original starting baseline.
If I type my salary or prices here, do you store them?
No — and that's deliberate. Everything runs locally in your browser, nothing leaves your device. We don't see your net salary, your margins, or the VAT numbers you test. For a finance tool that matters: you can run a quick check on a payslip or an invoice while on public Wi-Fi and close the tab knowing no trace is kept server-side.
Can I use it on the train without signal to check an invoice?
Yes. Load the page once, then it works offline. VAT, discount and mortgage math are all pure JavaScript — no API call. I often test it in airplane mode: open the mortgage page, type 250,000 at 4.5% for 25 years, you get the schedule instantly, even in a tunnel.
Can I file taxes with these results?
Use it as a fast sanity check, not as the final filing. Formulas are standard (e.g. Net = Gross / 1+VAT) and verified against official sources listed at the bottom, but rates and thresholds change — Italy's 22% vs Germany's 19%, or IRPEF brackets. For the actual declaration, double-check with your accountant or Agenzia delle Entrate / HMRC / IRS.
How do I send a calculation to my accountant?
Hit 'Share Link' under the results. It encodes the inputs in the URL — e.g. ?price=1200&vat=22 — so your accountant opens the exact same view, no screenshot needed. You can also copy the result with one tap and paste it into an email.