Calculate the elastic potential energy and force of a spring in compression or extension. U = ½·k·x², F = k·x. Solve energy from displacement, or displacement from energy.

Math & Numbers

Spring Rate Calculator

Calculate the stiffness (spring rate) of a cylindrical helical compression/extension spring. k = G·d⁴ / (8·D³·n) in N/mm. Includes shear-modulus presets for common spring materials.

Finance

APR Calculator

Calculate the true Annual Percentage Rate (APR) of a loan when upfront fees are involved. The nominal interest rate sets the monthly payment, but fees reduce the net cash received, raising the effective APR. Uses Newton's method to solve the annuity present-value equation. Shows APR, nominal rate, monthly payment, and the APR premium.

Finance

Currency Exchange Calculator

Convert an amount between currencies using live exchange rates from a free, no-key API (open.er-api.com). Supports 160+ world currencies. Shows the converted amount, the exchange rate used, a comparison table against popular currencies, and the rate update timestamp.

Finance

Salary Converter

Convert between hourly, monthly, and annual salary. Enter any one figure and get the other two, with customizable hours per week, weeks per year, and overtime multiplier. Shows paycheck estimates for weekly, biweekly, semimonthly, and monthly pay periods.

Finance

Investment Calculator

Project investment growth with initial principal plus monthly contributions and compound interest. Supports compounding frequency, inflation-adjusted real value, and yearly contribution escalation, with a yearly breakdown table and nominal vs real balance curves.

Finance

Present Value Calculator

Discount a future lump sum or an annuity back to today's value (PV). Supports lump-sum and annuity cash flows, multiple compounding frequencies, a present-value decay curve, and a per-period breakdown. PV = FV / (1+r)^n for a lump sum.

Finance

Retirement Calculator

Two-phase retirement model: an accumulation phase (current age to retirement, with monthly contributions and compound growth) and a withdrawal phase (retirement to life expectancy, nest egg grows at a conservative rate while inflation-adjusted withdrawals are taken out). Reports whether the nest egg lasts, real purchasing power, and any shortfall.

Finance

Savings Goal Calculator

Solve for the monthly contribution needed to reach a savings goal (with interest and compounding), or for the time required to reach the goal with a fixed monthly deposit. Optional inflation adjustment. Uses monthly compounding rollup.

Math & Numbers

Half-Life Calculator

Exponential decay for radioactive isotopes and drugs: remaining amount, derived half-life, time to target, or steady-state accumulation. Uses N(t) = N0·e^(-λt).

Math & Numbers

Pressure Calculator & Converter (P=F/A, Pa/kPa/bar/atm/psi)

Solve pressure, force, or area from P=F/A, and convert between Pa, kPa, MPa, bar, atm, psi, Torr, inHg, kgf/cm²

Math & Numbers

Significant Figures Sandbox

Practice significant-figure rules with step-by-step feedback: count sig figs, round to N sig figs, convert to scientific notation, and propagate through +/−/×/÷.