Simple Interest Calculator
Calculate simple interest with monthly precision, tax impact, inflation-adjusted value, target planning, and a side-by-side comparison against compound growth.
SI Calculator
📖 GuideAdvanced simple-interest planner
See after-tax value, real value after inflation, target planning, and simple vs compound comparison.
SI = (P × R × T) / 100. This version also shows monthly precision, after-tax value, real purchasing power, target planning, and simple vs compound difference.
Year-by-year breakdown
Simple total, after-tax value, and compound comparisonUnderstanding Simple Interest
What is Simple Interest?
Simple Interest (SI) calculates interest only on the original principal. The return grows linearly, not exponentially, which makes it easy to understand and predict.
Simple vs Compound Interest
Simple interest is easier to estimate, but compound interest usually wins over longer periods because it earns interest on earlier gains too.
Where SI Applies Best
Simple interest is common in short-term loans, informal lending, some debt contracts, and quick borrowing or lending decisions where clarity matters more than compounding.
Decision Map
When Simple Interest Works Best
Use this quick map to decide whether SI is enough or whether you should compare against compounding.
Short duration
Great for short fixed-tenure deals where you want a predictable amount quickly.
Negotiation clarity
Helpful when lending or borrowing informally and both sides want transparent math.
Watch inflation
If inflation is high, the purchasing power of the maturity value may disappoint.
Compare with compounding
For longer periods, always compare SI against compound interest before deciding.