Skip to content
Allin
Sign up · Sign in

Kepler's Third Law Calculator

Get the orbital period from T = 2π√(a³/GM) with the semi-major axis in AU, km or solar radii and the central mass in solar, Earth or Jupiter masses — plus orbital speed and the a³/T² constant.

Related tools

All Physics tools →
Coulomb's Law CalculatorSolve F = kq₁q₂/εᵣr² for the force, either charge or the distance, in any of 18 dielectric media, with charges in C, µC, nC or elementary charges, plus the potential energy and the field at q₂.Snell's Law CalculatorSolve n₁sin θ₁ = n₂sin θ₂ for either angle or either index, get the critical angle, and watch the incident, reflected and refracted rays move on a live diagram — total internal reflection included.Hooke's law calculatorCompute the force of a spring from its stiffness and displacement (F = kx).Spring constant calculator (Hooke's law)Hooke's law three ways: solve for the spring constant k = F/x, the restoring force F = k·x, or the displacement x = F/k. It handles single springs and N identical springs in series (k/N) or parallel (N·k), and adds the stored elastic energy ½kx².Ideal gas law calculatorCompute the pressure of a gas from moles, temperature and volume (PV = nRT).Ohm's Law CalculatorEnter any two of voltage, current, resistance and power and get the other two, with all 12 formula variants shown and an animated circuit whose charge carriers move with the current.Angular Velocity CalculatorConvert RPM, rad/s, Hz and period, or derive ω from a linear speed and radius — with tangential speed and centripetal acceleration.de Broglie Wavelength CalculatorCompute λ = h/p for an electron, proton, neutron, alpha particle, C₆₀ or any mass, from a velocity or a kinetic energy, using the relativistic momentum p = γmv.

Kepler's Third Law Calculator is free to use as often as you like, directly from this page. Its place is under Physics; Coulomb's Law Calculator and Snell's Law Calculator answer the questions closest to this one.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What is Kepler's Third Law Calculator?

Get the orbital period from T = 2π√(a³/GM) with the semi-major axis in AU, km or solar radii and the central mass in solar, Earth or Jupiter masses — plus orbital speed and the a³/T² constant.

When would I actually use this?

Checking a homework answer, sizing something before building it, and getting an order of magnitude before committing to a design — a torque on a bolt, the force a spring returns, the frequency a circuit resonates at, how long light takes to arrive.

What is the most common mistake?

Feeding in a value in the wrong unit. Physics formulas assume SI throughout, so grams instead of kilograms or centimetres instead of metres shifts the answer by powers of ten without any warning.

How is Kepler's Third Law Calculator different from Coulomb's Law Calculator?

They sit next to each other but answer different questions: Coulomb's Law Calculator is the one to open when you need it to solve F = kq₁q₂/εᵣr² for the force, either charge or the distance, in any of 18 dielectric media, with charges in C, µC, nC or elementary charges, plus the potential energy and the field at q₂. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

Snell's Law Calculator is the closest one after this: Solve n₁sin θ₁ = n₂sin θ₂ for either angle or either index, get the critical angle, and watch the incident, reflected and refracted rays move on a live diagram — total internal reflection included.

What else is worth having open alongside it?

Hooke's law calculator and Spring constant calculator (Hooke's law) — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

Constants are the CODATA values and the formulas are the textbook ones. Most assume an idealised case — no air resistance, no friction, a point mass — and the tool says so where the simplification matters.

Further reading

All guides →
ExplainerHooke's Law Explained: F = kx, Real Spring Constants, and Where It Stops HoldingHooke's law says force is proportional to stretch — but only below the elastic limit. Here is F = kx with worked numbers, what a 200 N/m spring actually feels like, and how springs combine.ExplainerThe Ideal Gas Law Explained: PV = nRT, R in Every Unit, and Where It BreaksPV = nRT holds when the gas is dilute and far from condensing. The value of R depends entirely on the units you feed it, and the temperature is never in degrees Celsius.ExplainerWhat Is Ohm's Law? V = I × R Explained SimplyOhm's law links voltage, current, and resistance with one equation: V = I × R. Learn the triangle trick and see worked examples.ExplainerResistors in Series and in Parallel: The Formulas, and the Three Things They Do Not Tell YouSeries adds, parallel adds reciprocals — that part is easy. The parts that catch people: which member dominates the answer, why two 5 % resistors in parallel are still 5 %, and why the smallest resistor in a parallel pair takes the largest share of the power.ExplainerThe Voltage Divider: The Formula Is Trivial, the Assumption Behind It Is NotVout = Vin·R2/(R1+R2) is only true when nothing is connected to the output. Draw current and the voltage collapses, by an amount that follows one exact law. Here is that law, the ten-times rule checked against it, and why a lower-resistance divider costs you power forever.ExplainerWhat Is Voltage Drop? Why Long Wires Lose VoltageVoltage drop is the voltage lost along a wire due to its resistance. Learn how length, gauge, and current affect it, and why it matters for your circuits.