How Interest Rate Calculator
Interest Rate Calculator works out Interest and Final amount from Principal, Time (years) and Time (extra months). The default figures on this page give annual rate of 5.0000 %; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
2 steps
- Term in years t = 2 + 0/12 + 0/365 = 2
- r = 100 x I / (P x t) = 100 x 100 / (1000 x 2) = 5%
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
2 sources
Keep this result
Save it, change one figure, then compare the two
The calculation stays free. Save this scenario, adjust one assumption, save that too, and put them side by side to see exactly what the change costs you. A free account keeps 5 scenarios and 2 comparisons; Personal makes both unlimited and adds exports and private links. Nothing is saved unless you press Save.
What the Interest Rate Calculator calculates
Computes simple, non-compounding interest and the final amount from a principal, an annual rate and a term given in years, months or days, and rearranges the same equation to solve for the rate, the principal or the time.
How to use this Interest Rate Calculator
Enter Principal, Time (years), Time (extra months) and Time (extra days). Interest updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Interest, Final amount and Principal.
The method follows OpenStax, Elementary Algebra 2e - simple interest and proportional relationships and US Treasury Financial Manual - ordinary (360-day) and exact (365-day) interest bases.
What you can compare
Change one input at a time to see how it affects Interest, Final amount and Principal. The inputs you can vary are Principal, Time (years), Time (extra months), Time (extra days), Days in a year and Interest amount.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do I calculate the interest rate?
Computes simple, non-compounding interest and the final amount from a principal, an annual rate and a term given in years, months or days, and rearranges the same equation to solve for the rate. Open “How this is worked out” beside the result to see each step for your own figures.
What is 5% interest on $5000?
Computes simple, non-compounding interest and the final amount from a principal, an annual rate and a term given in years, months or days, and rearranges the same equation to solve for the rate. On this page it is computed from Principal, Time (years) and Time (extra months) and shown as Interest, Final amount and Principal.
What is 2% interest of 20,000?
It needs Principal, Time (years), Time (extra months), Time (extra days), Days in a year and Interest amount. Nothing else is required, and the figures stay in your browser.
What information do I need for the Interest Rate Calculator?
You need Principal, Time (years), Time (extra months), Time (extra days), Days in a year and Interest amount. The page opens with default figures that give annual rate of 5.0000 %, so you can see the scale before entering your own.
What does this Interest Rate Calculator show?
It shows Interest, Final amount, Principal, Annual rate, Time, Interest per day and Interest per month, with the working and any warnings for your entries.
Can I check how the Interest Rate Calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.