The basic formula for percentage change

Percentage change tells you how much something has grown or shrunk as a portion of what it started at. The formula is: divide the difference (new amount minus old amount) by the old amount, then multiply by 100.

Here's the actual math: (New Value − Old Value) ÷ Old Value × 100 = Percentage Change. If the result is positive, the value went up. If it's negative, the value went down.

For homeowners, this matters when tracking property taxes, insurance premiums, repair costs, or your home's estimated value over time. The percentage tells you whether a change is actually significant or just noise.

Key Takeaways

  • Percentage change is calculated by subtracting the old value from the new value, dividing by the old value, and multiplying by 100.
  • A positive result means an increase; a negative result means a decrease.
  • Homeowners use this calculation to understand whether property tax hikes, insurance rate changes, or repair costs are reasonable compared to what they paid before.
  • The same formula works whether you're tracking a single year's change or comparing values across decades.
  • Percentage change is more useful than dollar amounts alone because it shows you the real impact relative to your starting point.

Working through a real homeowner example

Say your property tax bill was $3,200 last year and $3,520 this year. The difference is $320. Divide $320 by $3,200 (the old amount) to get 0.1. Multiply by 100 and you get 10%. Your property tax increased by 10%.

Now imagine your homeowners insurance premium went from $1,400 to $1,470. The difference is $70. Divide $70 by $1,400 to get 0.05. Multiply by 100 and you get 5%. Your insurance went up 5%.

Both are increases, but the property tax jump is steeper. A 10% increase on a $3,200 bill is harder to absorb than a 5% increase on a $1,400 bill. Percentage change lets you compare apples to apples, even when the dollar amounts are different.

When your home's value changes

If your home was assessed at $250,000 five years ago and a new assessment puts it at $290,000, you can calculate the percentage change. The difference is $40,000. Divide $40,000 by $250,000 to get 0.16. Multiply by 100 and you get 16%. Your home's assessed value increased 16% over that period.

This matters because higher assessed value often means higher property taxes, even if the tax rate stays the same. It also matters when you're deciding whether to refinance or sell — a 16% increase over five years tells you something about your local market that a raw dollar figure alone does not.

Keep in mind that assessed value and market value are not the same thing. Your county assessor's number is what determines your tax bill. A real estate agent's estimate or a home valuation website gives you a different picture. Calculate the percentage change for whichever number you're tracking, but know which one you're using.

Handling negative changes and decreases

If a value goes down, the math works the same way — the result will just be negative. Say your home was assessed at $300,000 and dropped to $270,000. The difference is −$30,000. Divide −$30,000 by $300,000 to get −0.1. Multiply by 100 and you get −10%. Your assessed value decreased 10%.

A negative percentage change can actually be good news if you're tracking costs. If your homeowners insurance premium dropped from $1,600 to $1,440, that's a −$160 change. Divide −$160 by $1,600 to get −0.1, which is −10%. Your premium fell 10%, which saves you money.

Comparing changes across different time periods

Percentage change works whether you're looking at one year or ten years. The formula does not care about the time span — it only measures the gap between start and finish.

If your property tax was $2,800 ten years ago and is $3,520 now, the difference is $720. Divide $720 by $2,800 to get 0.257. Multiply by 100 and you get 25.7%. Over a decade, your property tax increased roughly 26%.

That 26% tells you something useful: your tax bill has grown faster than inflation typically does. It also gives you a baseline if you want to estimate what you might owe in another ten years, though future increases depend on local policy and your home's changing assessment.

Why percentage change matters more than the dollar amount alone

A $100 increase sounds small until you know what it's a percentage of. A $100 increase on a $1,000 bill is 10%. A $100 increase on a $10,000 bill is 1%. The same dollar amount means very different things depending on the starting point.

For homeowners, this is especially important when comparing your costs to what others pay or to what you paid in previous years. If your neighbor's insurance went up $50 and yours went up $200, you might think yours increased more. But if your neighbor's premium started at $500 and yours started at $2,000, your percentage increase might actually be lower.

Percentage change also helps you spot trends. If your property tax has increased 3% every year for five years, you can reasonably expect another 3% increase next year and plan your budget accordingly. A raw dollar amount does not show you that pattern as clearly.

Common mistakes to avoid

The most common error is reversing the order — subtracting the new value from the old value instead of the other way around, or dividing by the new value instead of the old value. Both will give you the wrong sign or the wrong magnitude. Always subtract the old from the new, and always divide by the old.

Another mistake is forgetting to multiply by 100. If you get 0.15 from your division, that is 15%, not 0.15%. The multiplication by 100 converts the decimal to a percentage.

A third pitfall is using the wrong starting point. If you want to know how much your home's value changed from 2015 to 2024, use the 2015 value as your old amount, not the 2024 value. The starting point is always the baseline you're measuring from.

Frequently Asked Questions

Can I use this formula for things other than home costs?

Yes. The percentage change formula works for any two numbers — repair costs, square footage, interest rates, or anything else. As long as you have a starting value and an ending value, you can calculate the percentage change between them.

What if the old value was zero?

You cannot divide by zero, so the formula breaks down. If something went from zero to a positive number, there is technically an infinite percentage increase. In practical terms, you would just describe it as "new" rather than trying to calculate a percentage.

Should I round my answer?

For most homeowner purposes, rounding to one or two decimal places is fine. A 10.3% increase and a 10.27% increase mean roughly the same thing for budgeting or comparison. Round to whatever precision makes sense for your situation.

Does the time period affect how I calculate percentage change?

No. The formula is the same whether you are comparing values one month apart or ten years apart. The time span does not change the math, though it might change how you interpret the result — a 5% annual increase is steeper than a 5% increase over five years.

How do I calculate average percentage change over multiple years?

That requires a different formula called the compound annual growth rate (CAGR), which is more complex. For most homeowner tracking, calculating the total percentage change from start to finish is enough. If you need year-by-year breakdowns, calculate the percentage change for each individual year separately.