Open Money Tools
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✦ Free & Instant — No Signup Required

Interest calculators
for every decision

Whether you're saving, investing, or borrowing — get accurate numbers in seconds and understand exactly what they mean.

4
Calculators
Free
Always
0
Signups needed
Instant
Results
Our calculators

Pick your calculator

Each tool is built for a specific financial question — choose the one that fits your situation.

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Simple Interest

Calculate interest on the original principal amount only — ideal for short-term loans and deposits.

  • Enter principal, rate, and time period
  • Instant interest and total value
  • Works in GBP, USD, EUR, and more
  • Clear formula explanation included
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Compound Interest

See how money grows exponentially when interest is earned on both principal and accumulated gains.

  • Choose compounding frequency
  • Year-by-year breakdown table
  • Compare simple vs compound side-by-side
  • Growth factor and effective rate
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Savings Growth

Project your savings with an initial lump sum plus regular monthly contributions over time.

  • Lump sum + monthly contributions
  • Milestone projections (1, 5, 10, 20 yrs)
  • See how much is interest vs deposits
  • ISA and pension goal planning
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Loan Interest

Find out the true cost of borrowing — monthly repayments, total interest, and full loan cost.

  • Repayment and interest-only options
  • Monthly payment breakdown
  • Total interest vs principal visual
  • Suitable for mortgages, personal loans

Why use Open Money Tools?

Built to be fast, honest, and actually useful

Instant results
No page reloads or waiting. Numbers update as you type, so you can explore scenarios quickly.
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Explained clearly
Every calculator comes with plain-English explanations of the maths, so you actually understand your result.
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No data collected
All calculations happen in your browser. We never see the numbers you enter, ever.
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Works everywhere
Fully responsive on desktop, tablet, and mobile — no app download needed.
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Purpose-built tools
Each calculator is designed for one specific question, so you get the exact result you need.
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UK-focused
Default rates and examples reflect UK savings, mortgage, and loan market conditions.

Compounding is the most powerful force in personal finance

Time is the single biggest factor in how much your money grows. Starting earlier — even with a smaller amount — consistently beats starting later with more.

Our calculators show you exactly how this plays out, year by year, so you can see the real cost of waiting.

  • £10,000 at 7% for 10 years grows to ~£19,700
  • The same money for 20 years reaches ~£38,700
  • Starting 5 years earlier can add 40–60% to your final balance
£10,000 at 7% — compound growth
Yr 0
£10k
Yr 5
£14k
Yr 10
£19.7k
Yr 15
£27.6k
Yr 20
£38.7k
Common questions

FAQ

Yes, completely free. No account, no subscription, no tricks. All four calculators are available to anyone at any time.

The results use standard mathematical formulas and are arithmetically accurate. Real-world figures may differ due to variable rates, fees, tax implications, and payment timing. Always consult a qualified financial adviser before making major decisions.

Use the Loan Interest Calculator — it handles both repayment (capital + interest) and interest-only mortgages and shows your monthly payment, total interest, and full cost over the term.

Compound interest is almost always better for savings — you earn interest on your accumulated interest, not just the original deposit. The longer your time horizon, the bigger the difference. Use our Compound Interest Calculator to see this in action.