About this calculator
Two electric charges push or pull on each other. Coulomb's law gives the size of that force: F = k q₁ q₂ ÷ r².
Choose the charge unit (coulombs, millicoulombs, microcoulombs or nanocoulombs), enter both charges and the distance in metres. Use a minus sign for a negative charge. The calculator tells you whether the force attracts or repels.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
Two 1 µC charges 1 m apart
- Force between the charges
- 8.98755 mN (8.98755 × 10^-3 N)
- Type of force
- Repulsive: like charges push apart
- Electric field of charge 1 at that distance
- 8.98755 × 10^3 N/C
Two charges of 1 and 1 µC, 1 m apart, repel each other with a force of 8.98755 mN.
Show the working
- Coulomb's law: F = k × q₁ × q₂ ÷ r², where k = 8.9875517923 × 10⁹ N·m²/C².
- F = 8.98755 × 10^9 × |1 × 10^-6 × 1 × 10^-6| ÷ 1² = 8.98755 × 10^-3 N.
- The force falls with the square of the distance: double the distance and it is four times weaker.
+1 µC and −1 µC, 0.5 m apart
- Force between the charges
- 35.9502 mN (3.59502 × 10^-2 N)
- Type of force
- Attractive: opposite charges pull together
- Electric field of charge 1 at that distance
- 3.59502 × 10^4 N/C
Two charges of 1 and -1 µC, 0.5 m apart, attract each other with a force of 35.9502 mN.
Show the working
- Coulomb's law: F = k × q₁ × q₂ ÷ r², where k = 8.9875517923 × 10⁹ N·m²/C².
- F = 8.98755 × 10^9 × |1 × 10^-6 × -1 × 10^-6| ÷ 0.5² = 3.59502 × 10^-2 N.
- The force falls with the square of the distance: double the distance and it is four times weaker.
Two 5 nC charges 2 cm (0.02 m) apart
- Force between the charges
- 561.722 µN (5.61722 × 10^-4 N)
- Type of force
- Repulsive: like charges push apart
- Electric field of charge 1 at that distance
- 1.12344 × 10^5 N/C
Two charges of 5 and 5 nC, 0.02 m apart, repel each other with a force of 561.722 µN.
Show the working
- Coulomb's law: F = k × q₁ × q₂ ÷ r², where k = 8.9875517923 × 10⁹ N·m²/C².
- F = 8.98755 × 10^9 × |5 × 10^-9 × 5 × 10^-9| ÷ 0.02² = 5.61722 × 10^-4 N.
- The force falls with the square of the distance: double the distance and it is four times weaker.
The formula
F = k × q₁ × q₂ ÷ r², where k = 8.9875517923 × 10⁹ N·m²/C² (about 9 × 10⁹). The force is in newtons when the charges are in coulombs and the distance in metres.
Attract or repel?
Like charges repel (two positives or two negatives) and opposite charges attract. The size of the force is the same either way: only the direction changes.
Why a coulomb is huge
One coulomb is an enormous charge. Two 1 C charges 1 metre apart would push with about 9 billion newtons. Everyday static electricity involves microcoulombs or nanocoulombs, so the forces are tiny: a few millinewtons at most.
Frequently asked questions
What does Coulomb's law say?
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance.
What is the value of k?
About 8.99 × 10⁹ N·m²/C², also written 1 ÷ (4π ε₀).
What happens if I double the distance?
The force becomes four times smaller.
Is the force attractive or repulsive?
Repulsive for like charges, attractive for opposite charges. The calculator tells you.
Does it work for charged spheres?
Yes, if you measure the distance between their centres, for uniformly charged spheres.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.