Interest Calculator
Compare simple and compound interest on the same principal, at any compounding frequency, and see precisely what the frequency alone is worth over the life of the deposit.
Interest Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: Simple interest is I = P·r·t and accrues only on the original principal. Compound interest is A = P(1 + r/n)^(nt) and accrues on interest already earned. On $25,000 at 6.5% for ten years, simple interest pays $16,250.00 while monthly compounding pays $22,804.59 — a $6,554.59 difference created entirely by the compounding frequency.
Formula
P = principal · r = annual rate · n = compounding periods per year · t = years
Worked Example
- Set the periodic rate. 6.5% ÷ 12 = 0.0054166 per month.
- Count the periods. 12 × 10 = 120 compounding periods.
- Grow the principal. $25,000 × (1.0054166)120 = $47,804.59.
- Isolate the interest. $47,804.59 − $25,000.00 = $22,804.59.
- Compare with simple interest. $25,000 × 6.5% × 10 = $16,250.00, so compounding is worth $6,554.59.
Analyst's note. The compounding advantage here is 40.3% of the simple-interest figure, and not one basis point of extra rate was required to earn it. Note also the ceiling: continuous compounding pays $22,888.52, only $83.93 more than monthly. Frequency matters enormously between annual and monthly, and almost not at all beyond daily.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Choosing between deposit products
Two accounts quoting the same nominal rate are not the same account. Compare them on effective annual rate, which folds the frequency in: 6.5% compounded monthly is 6.697% effective. The APY Calculator converts any nominal rate to its annual equivalent yield for a like-for-like comparison.
Pricing a fixed-term deposit or bond
Many instruments accrue simple interest and pay it away rather than reinvesting it. Set the frequency to annual and compare against the simple figure to see the reinvestment risk you carry. The Simple Interest Calculator isolates the linear case, and the Bond Yield Calculator handles coupons.
Understanding the cost of a borrowing
The same arithmetic runs in reverse against you on credit. A balance compounding monthly at a card rate grows on exactly this curve, which is why minimum payments extend so far. Model the liability with the Credit Card Payoff Calculator instead of assuming the quoted rate is the annual cost.
Methodology & Editorial Standards
Compound growth uses the closed-form A = P(1 + r/n)^(nt) evaluated in double precision, with the continuous case computed as Pe^(rt) to show the theoretical ceiling of frequency. Simple interest is computed independently as P·r·t rather than derived from the compound figure, so the two are genuinely separate calculations and the reported advantage is a true difference. The effective annual rate is (1 + r/n)^n − 1, and time-to-double is solved analytically from the periodic rate rather than approximated by the rule of 72, which the tool reports for reference only. The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Interest Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is the difference between simple and compound interest?
Simple interest accrues only on the original principal, so it is linear: the same amount every year. Compound interest accrues on the principal plus all interest already credited, so it is exponential. On $25,000 at 6.5% for ten years the difference is $16,250.00 against $22,804.59.
How does compounding frequency change the result?
More frequent compounding credits interest sooner, so it starts earning sooner. The same 6.5% pays $21,928.44 compounded annually and $22,885.75 compounded daily on this deposit. The gain from annual to monthly is large; from daily to continuous it is trivial.
What is the effective annual rate?
The rate that, compounded once a year, produces the same result as the nominal rate compounded n times. It is (1 + r/n)^n − 1. It exists so that products with different compounding frequencies can be compared on one number, and it is the only fair basis for such a comparison.
Is continuous compounding worth chasing?
No. It is the mathematical ceiling of frequency, computed as Pe^(rt), and on this deposit it pays $22,888.52 against monthly compounding's $22,804.59 — $83.93 over a decade. It matters in derivatives pricing, not in deposit selection.
How long will it take my money to double?
At 6.5% compounded monthly, 10.69 years. The rule of 72 estimates 11.1 years, which is close enough for mental arithmetic but wrong by five months here. This tool solves it exactly with logarithms.
Does the calculator handle contributions?
This engine models a single lump sum so that the simple-versus-compound comparison stays clean. For recurring deposits use the Compound Interest Calculator or the Savings Calculator, both of which add a contribution stream to the same growth mathematics.
Which frequency do banks actually use?
US deposit accounts commonly compound daily and credit monthly; certificates often compound monthly or quarterly; many bonds pay simple semi-annual coupons with no compounding at all unless you reinvest. Always check the disclosure rather than assuming.
Is the interest I earn taxable?
In most jurisdictions, yes, in the year it is credited — even if you do not withdraw it. This engine reports gross interest. Your after-tax return is materially lower, and tax paid annually also removes the compounding on that amount.
Why does the daily figure look so small?
Because it is one day of the first year only: $4.59 here. Daily accrual grows as the balance grows, so the final year's daily figure is far larger. It is shown to make the accrual mechanism tangible, not as an average across the term.
Can I use this for a loan instead of a deposit?
The mathematics is identical but the direction is reversed, and most loans amortise — you make payments that reduce the balance. For borrowings use the Loan Calculator or the Payment Calculator, which model the repayment schedule rather than uninterrupted growth.
What rate should I assume?
Use the rate you are actually offered, not a market average. If you are projecting rather than pricing a specific product, model a range: the difference between 4% and 6.5% over a decade is far larger than most people intuit.
Does inflation affect these figures?
Substantially. Interest reported here is nominal. At 3% inflation, $47,804.59 in ten years has roughly the purchasing power of $35,570 today. Use the Inflation Calculator to convert any projection into constant purchasing power before judging it.
Why is my bank's quoted APY different from my rate?
APY already includes compounding; the nominal or 'interest rate' does not. A 6.5% nominal rate compounded monthly is quoted as 6.697% APY. If a bank quotes you one number, establish which it is before comparing.
Does the term have to be a whole number of years?
The term box accepts whole years by design, since the compounding grid is annual-based. For part-year or month-precise projections, the Future Value Calculator and the Savings Calculator both work on a monthly grid.
What happens if the rate is zero?
Both figures collapse to zero interest and the balance equals the principal, which is the correct result rather than an error. Time-to-double is undefined at a zero rate and shows an em-dash.
Is this interest calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.