A Compound vs Simple Interest computes interest accrued on a principal at a fixed rate. It applies the standard formula to the values you enter and returns the result instantly, without sending any data to a server. Free Compound vs Simple Interest.
Interest calculated only on the principal (no compounding).
Simple interest I = P x r x t. Compound interest (annual) A = P x (1+r)^t.
Compound: interest on interest. Simple: only on principal.
Simple interest: interest only on original principal. Compound interest: interest on principal + accumulated interest. Over long periods, compound dramatically outpaces simple. At 7% over 30 years, compound interest produces ~7.6× the principal vs simple ~3.1×.
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
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Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
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Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.
The 4% rule: you can withdraw ~4% of your portfolio annually with high confidence of lasting 30+ years. So if you need $50K/year, target $1.25M. The rule was developed for 30-year retirements in the US - for 40+ years (early retirement) use 3-3.5%.
Over 1-year periods: very volatile, ~30% historical loss possible. Over 10-year periods: 95% positive historically. Over 30-year periods: 100% positive in any rolling US window. Risk depends on time horizon, not the asset class itself.
Compare your mortgage rate to expected investment return. If mortgage rate is below 5% and your retirement contributions are maxed, investing usually wins long term. Above 7%, the guaranteed return from paying off the mortgage often wins.
Rough target: 15-20% of gross income toward retirement, starting at 25. If you start at 35, you need 25-30%. At 45, 40%+. Saving rate matters more than investment selection for the first 10-15 years.
Short-term government bonds in your home currency. Inflation-linked bonds (TIPS US, ILBI India, index-linked gilts UK) protect against inflation. Bank savings accounts up to insured limits ($250K US FDIC, £85K UK FSCS) are also safe but lose to inflation.
Compounding is the engine of every long-term investment plan. The formula for monthly contributions:
FV = PMT x ((1+r)^n - 1) / r
where PMT = monthly contribution, r = monthly return rate, n = months. With a one-time lump sum P at the start: FV += P x (1+r)^n.
Saving $500/month from age 25 to 65 (40 years) at 8% reaches ~$1.75M. Same $500/month starting at 35 reaches only ~$745K. The 10 extra years more than DOUBLES the final balance - that's the difference between compounding for 30 vs 40 years.
Charlie Munger's observation: getting to $100K is brutal because you depend on contributions, not returns. After $100K, returns start to do more work than your savings. After $1M, your annual return often exceeds your annual contribution.
| Country | Pre-tax (defer) | After-tax (Roth-style) | Annual limit (2026) |
|---|---|---|---|
| US | 401(k), Traditional IRA, HSA | Roth IRA, Roth 401(k) | $23,500 401(k) / $7,000 IRA / $4,300 HSA |
| UK | Workplace pension, SIPP | ISA (Stocks/Cash/LISA) | £60,000 pension annual / £20,000 ISA |
| Canada | RRSP, FHSA | TFSA | 18% income RRSP / $7,000 TFSA / $8,000 FHSA |
| Australia | Super (concessional) | Super (non-concessional) | AUD 30,000 concessional / AUD 120,000 non-conc |
| India | EPF, NPS, PPF | Equity LTCG (limited) | Rs 1.5L 80C / Rs 50K NPS / Rs 1.5L PPF |
| Singapore | SRS | CPF top-ups | SGD 15,300 SRS (Singaporean) / 35,700 (foreigner) |
| Germany | Riester, Rürup, bAV | Few options | EUR 29,344 Rürup max |
The classic rule "100 minus your age in stocks" is too conservative for modern lifespans. Updated guidance:
| Age | Equity % | Bonds % | Cash % |
|---|---|---|---|
| 25-35 | 90-100% | 0-10% | 0% |
| 35-45 | 80-90% | 10-20% | 0% |
| 45-55 | 70-80% | 15-25% | 5% |
| 55-65 | 55-70% | 25-35% | 5-10% |
| 65+ (retired) | 40-60% | 30-50% | 10% |
If you retire at 65 and live to 90, your retirement portfolio still has 25-year horizon. Too conservative an allocation runs out of money. Too aggressive risks sequence-of-returns disasters early in retirement.
This calculator uses the following formula:
Simple: A = P(1 + rt); Compound: A = P(1 + r/n)^(nt)
The reason this formula works is rooted in the underlying physics, finance, or biology of the problem. Behind every calculator is a published, peer-reviewed equation or a widely accepted convention. We do not invent formulas; we apply standard ones from textbooks, government tables, professional bodies, and academic literature.
If you are curious about the math, the simplest way to verify is to plug in two known numbers and compare against a known result. The calculator should match published examples to within rounding precision.