Interest Month Calculator
Monthly Interest Calculator works out Interest and Final amount from Principal, Annual interest rate and Time (extra months). The default figures on this page give interest of 4.17; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
3 steps
- Term in years t = 0 + 1/12 + 0/365 = 0.08333333
- I = P x (r/100) x t = 1000 x (5/100) x 0.08333333 = 4.166667
- A = P + I = 1000 + 4.166667 = 1004.166667
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 Monthly Interest 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 Monthly Interest Calculator
Enter Principal, Annual interest rate, Time (extra months) and Days in a year. 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, Annual interest rate, Time (extra months) and Days in a year.
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 interest per month?
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.
How do I calculate interest for one month?
It needs Principal, Annual interest rate, Time (extra months) and Days in a year. Nothing else is required, and the figures stay in your browser.
Is 1% per month the same as 12% per year?
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 the figures this page opens with — principal 1,000 and annual interest rate 5 percent/year — the interest is 4.17. Put your own figures in their place and the working updates beside the result.
How is interest calculated each month?
The result gives Interest, Final amount, Principal, Annual rate, Time and Interest per day, each labelled with its unit so the figure can be read on its own.
How do I calculate my monthly interest amount?
The page opens with worked default figures: interest of 4.17. Replace them with your own and every figure updates as you type.
What information do I need for the Monthly Interest Calculator?
You need Principal, Annual interest rate, Time (extra months) and Days in a year. The page opens with default figures that give interest of 4.17, so you can see the scale before entering your own.