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Surface gravity calculator

Compute the surface gravity of a planet or moon from its mass and radius.

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Need Surface gravity (m/s²)? The Surface gravity calculator derives it from Mass (×10²⁴ kg), Radius (km) in one step. For instance, with Mass (×10²⁴ kg) = 5.972 and Radius (km) = 6,371 it returns Surface gravity (m/s²) = 9.82.

How to use it

  1. Enter your values: Mass (×10²⁴ kg), Radius (km).
  2. Read the result instantly: Surface gravity (m/s²).

Frequently asked questions

What does the Surface gravity calculator actually compute?

It takes Mass (×10²⁴ kg) and Radius (km) and derives Surface gravity (m/s²) from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

2 values: Mass (×10²⁴ kg) and Radius (km). Nothing else is required — no account, no file upload.

Can you show a worked example?

With Mass (×10²⁴ kg) = 5.972 and Radius (km) = 6,371, the calculator returns Surface gravity (m/s²) = 9.82. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Mass (×10²⁴ kg) = 11.944 and Radius (km) = 7,008 instead, Surface gravity (m/s²) goes from 9.82 to 16.231 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Mass (×10²⁴ kg) = 2.986 and Radius (km) = 5,734, Surface gravity (m/s²) comes out at 6.061. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Planning an observation or a photograph: when the sun sets, what phase the moon is in, and when an equinox or a solstice actually falls this year.

What is the most common mistake?

Treating the equinox as a fixed calendar date. It drifts by up to two days because the tropical year is not a whole number of days, which is the same reason leap years exist.

What is the difference between the Surface gravity calculator and the Falling through the Earth calculator (gravity tunnel)?

This one returns Surface gravity (m/s²); the Falling through the Earth calculator (gravity tunnel) returns One-way fall time and Full oscillation (round trip). That is the whole difference — open the one whose figure you need.

Where do the figures come from, and how current are they?

Positions are computed from published ephemeris algorithms (Meeus) including the correction for the Earth's irregular rotation, so results agree with an almanac to well under a minute for the coming decades.

Further reading

All guides →
ExplainerHow Moon Phases Work: Two Months, Eight Shapes, and No ShadowThe phase cycle takes 29.53 days but the orbit takes only 27.32, and the 2.21-day gap is the entire explanation. With the eight phases, their illumination, when each rises and sets, and how they flip south of the equator.ExplainerThe Golden Hour and the Blue Hour Are Angles, Not HoursGolden hour runs from +6° to −4° of solar elevation and blue hour from −4° to −6°, which is why it lasts forty minutes at the equator, over an hour at mid-latitudes, and above 72.6° in June does not happen at all. The thresholds this calculator uses, checked against its own output.ComparisonWhich Fishing Knot Holds Best, and Why Every Published Table DisagreesKnot strength is quoted as a percentage of the line's rated breaking strain, and the percentages in circulation come from tests nobody agrees on. What the calculator on this page really stores, what one peer-reviewed measurement actually found, where a line breaks and why, and how to match a knot to its job.ExplainerDouble the Drop and the Speed Rises 41 %, but the Energy DoublesA 20-metre fall takes 2.02 seconds and lands at 19.81 m/s. From 40 metres it takes 2.86 seconds and lands at 28.01 m/s — the speed is up by a factor of √2, and the energy by exactly two.ExplainerFriction Does Not Care How Big the Contact Patch IsA coefficient of 0.3 against a normal force of 100 N gives 30 N of friction, and the area never enters the formula. What does enter it, and is usually got wrong, is the normal force.ExplainerTen Metres of Copper Pipe Grows Eight MillimetresA straight run of copper going from 20 to 70 degrees gains 8.5 mm over ten metres. Steel gains 6 mm, and a 100-metre steel span over a 60-degree year moves 72 mm.