Financial calculators

The classic formulas,
on sliders.

Compound interest, Rule of 72, CAGR, inflation, mortgages, present value, retirement drawdown. Drag a slider, watch the number move.

Compound interestRule of 72CAGRReal return / inflationMortgage / loanPresent valueRetirement drawdownDoubling time
Growth

Compound interest

The 8th wonder of the world. Watch a starting sum plus monthly additions balloon over time — the interest overtakes contributions.

Compounding
BalanceContributions only
Invested
$130,000
Interest
$170,851
Final
$300,851
Formula: FV = P(1 + r/n)^(nt) + PMT · [((1 + r/n)^(nt) − 1) / (r/n)]
Shortcut

Rule of 72

Divide 72 by your annual return to estimate how long money takes to double. A mental-math trick built on ln 2.

Divide 72 by your rate to estimate how long money takes to double. Works well for rates roughly between 4% and 12%. Below that use 69; above, use 76.
Rule of 72 (approx)
10.3 yrs
Exact (ln 2 / ln(1+r))
10.2 yrs
Doublings in 30 yrs
2.9
$1 → after 30 yrs
$8
$1 doubling ladder
$1
yr 0
$2
yr 10
$4
yr 21
$8
yr 31
$16
yr 41
$32
yr 51
$64
yr 62
$128
yr 72
Backwards look

CAGR

Compound Annual Growth Rate — the smooth annualised return between two dated values, ignoring the bumps in between.

Total return
150.00%
CAGR
13.99%
Multiple
2.50×
Simple annual
21.43%
CAGR strips out the noise: the constant rate that would compound your starting value into your ending value over the period. Total return divided by years overstates it (ignores compounding).
Purchasing power

Real return / inflation

Nominal returns lie. Strip out inflation to see what your money actually buys in the future.

Real return
4.85%
Rule-of-thumb
5.00%
Nominal FV
$466,096
Real FV (today's $)
$258,066
Cash under mattress loses
$44,632
Same purchasing power?
$180,611
Real ≈ Nominal − Inflation is the shortcut. Exact: (1+r)/(1+i) − 1. The gap matters at high inflation — 10% nominal & 8% inflation is only 1.85% real, not 2%.
Debt

Mortgage / loan

Amortising loan calculator. Monthly payment, total interest, and how the balance winds down over the term.

Monthly payment
$1,799
Total paid
$647,515
Interest cost
$347,515
Balance over time
Amortising loan: fixed monthly payment, mostly interest early, mostly principal late. Halving the term more than halves total interest.
Time value

Present value

What a future sum is worth today at a given discount rate. Foundation of every DCF.

Present value
$25,842
Discount vs FV
$74,158
PV as % of FV
25.84%
Discount factor
0.2584
PV = FV / (1 + r)ⁿ. Every future cash flow is worth less today — because you could invest a smaller sum today at rate r and get to the same place.
Retirement

Retirement drawdown

Pot, withdrawal rate, growth, inflation — the four levers behind the 4% rule. Watch survivability in real time.

Balance over years (nominal)
First-year withdrawal
$40,000
Real return
3.41%
Balance after 30 yrs
$1,576,721
Withdrawal @ end
$175,992
The 4% rule (Trinity Study) says 4% of the starting pot, adjusted for inflation each year, historically survived 30 years. Try 5% or 6% — survivability drops fast.
Multiplier

Doubling time

A different view on compounding: at rate r over t years, how many doublings do you get? Small rate changes, big final differences.

Multiplier
7.61×
Final value
$76,123
Times doubled
2.93
Years per double
10.2