Convert an Earth scale reading or a mass into what you would weigh on the planets and Moon using clear units and gravity values.
Advanced options
Definitions
Earth scale reading: The number a scale shows you on Earth, usually in lb or kg. A scale reading changes with local gravity because the scale measures how hard gravity pulls on you.[2]
Mass: The amount of matter in an object. Your mass stays the same on every planet and moon.
Weight force: The force of gravity pulling on your mass. In physics, weight is a force, not just a scale number.[2]
Newton (N): The SI unit of force. In Mass mode, the calculator can show your true weight force in newtons.
Surface gravity: The gravitational pull near the surface of a world, measured here in m/s^2. Larger surface gravity means a larger weight force for the same mass.
Standard gravity: A defined reference value for Earth gravity of 9.80665 m/s^2, often used in science and engineering.[1]
Earth check: A quick comparison row. In Earth scale mode, Earth should return the same number you entered before rounding.
Significant figures: The number of meaningful digits shown in a result. They control rounding but do not change the underlying calculation.
Common mistakes and quick fixes
Mistake: Entering a value in Mass (selected unit) while the mode is set to What do you want to enter? = Earth scale reading, then expecting that mass to affect Weights on each world .
Fix: If you want physics weight from mass, switch What do you want to enter? to Mass (kg or lbm). In Earth scale mode, only Earth scale reading (selected unit) and Earth scale unit are used.
Mistake: Typing kg into Earth scale reading (selected unit) but leaving Earth scale unit on lb.
Fix: Match the number to the correct Earth scale unit . If your scale says kilograms, choose kg. This also helps the Earth check (should match your Earth input) make sense.
Mistake: Using pounds of body weight as if they were mass in Mass (selected unit) without changing Mass unit to lbm.
Fix: In Mass mode, choose the real mass unit you entered. If you typed pounds-mass, set Mass unit to lbm so Mass (kg) used in calculations is converted correctly.
Mistake: Leaving Show weight force (newtons) off, then wondering why there is no force value in Weights on each world .
Fix: Turn on Show weight force (newtons) if you want the physics force in N. If it stays off, read the Unit note so you know the results are scale-style readings only.
Mistake: Setting Significant figures (count of digits) too low or too high and misreading rounded results as exact.
Fix: Use 3 to 5 significant figures for most school work. If the formatting looks odd, adjust Number display format and recheck Weights on each world .
Mistake: Thinking a different value in Earth gravity value to use for ratios changes your mass.
Fix: That setting only changes ratio calculations in Earth scale mode. It does not change Mass (kg) used in calculations in Mass mode.
Limitations & Key Assumptions / Boundary Conditions
- The calculator uses rounded educational surface gravity values for Mercury, Venus, Earth, the Moon, Mars, Jupiter, Saturn, Uranus, Neptune, and optional Pluto, so small differences from textbooks or agency tables are normal.
- Earth scale mode assumes a similar scale would change in direct proportion to local gravity. That is a useful classroom model, but real scales and environments can behave differently.
- Mass mode treats weight as W = m times g and assumes you are standing still near the surface of the selected world.
- The Earth gravity choice in Advanced options affects ratio results only in Earth scale mode. It does not change your mass.
- Results depend on the unit you choose. lb and kg in Earth scale mode are treated as scale-display units, while lbm in Mass mode is converted to kg before force is computed.
- Inputs must be greater than 0. Blank, zero, or negative values are input errors and should be corrected before using the results.
- Rounding and scientific notation only change how numbers are displayed. They do not change the actual values used internally.
Methodology
Calculation method
The calculator has two modes because a scale reading and a physics force are not exactly the same idea. NASA explains that when you stand on a scale, it is measuring how hard gravity pulls on you, so your weight changes on other worlds when gravity changes.[2]
W = m * g
In Mass mode, the calculator converts your mass to kilograms if needed, then multiplies by each world's surface gravity to get weight force in newtons.
Reading_world = Reading_earth * (g_world / g_earth_used)
In Earth scale mode, the calculator keeps your input unit the same and scales the Earth reading by the gravity ratio for each world. The Earth row should match your input exactly before formatting.
m_kg = m_lbm * 0.45359237
If you enter lbm in Mass mode, the calculator first converts pound-mass to kilograms, then applies the force formula.
Gravity values used
The world-by-world comparison uses a rounded set of surface gravity values commonly used for classroom work. The optional Earth reference for ratio mode can be either 9.8 m/s^2 or the standard gravity value 9.80665 m/s^2.[1]
Worked mini-example
Suppose you choose Earth scale mode, enter 150 lb, and keep Earth gravity at 9.8 m/s^2. On Mars, using 3.7 m/s^2, the scale-style reading is 150 * (3.7 / 9.8) = about 56.6 lb. On Jupiter, using 23.1 m/s^2, the reading is 150 * (23.1 / 9.8) = about 353.6 lb.
Now suppose you choose Mass mode and enter 68 kg. On Earth, weight force is 68 * 9.8 = 666.4 N. On the Moon, weight force is 68 * 1.6 = 108.8 N. If scale-style readings are also shown, they are derived from those force differences relative to Earth gravity.
Formatting and checks
The display settings only change how numbers look. In Auto mode, very large or very small values may switch to scientific notation, while ordinary values stay in standard form. If significant figures are blank or outside the allowed range, the calculator falls back to 4 and notes that change in the unit note.
Assumptions used
This tool is meant for learning and quick comparisons, not for modeling every real-world detail. It assumes near-surface gravity, ignores motion and local variations, and uses rounded gravity values, so exact results can differ slightly from other references or devices.