What is the Coulomb constant k?

The Coulomb constant k (also written ke) equals 8.9875 × 10⁹ N·m²/C², derived from 1/(4πε₀) where ε₀ is the permittivity of free space.

What does a negative force mean?

A negative result indicates an attractive force (opposite-sign charges). A positive result indicates a repulsive force (same-sign charges).

Does this apply to non-point charges?

Coulomb's Law is exact only for point charges. For extended bodies it is a good approximation when the separation is much larger than the object size.

What units should I enter charges in?

Select your preferred unit from the dropdown (C, mC, µC, nC, pC). The calculator converts automatically before applying the formula.

What if I enter zero for a value?

If either charge is zero the force is zero. If distance is zero the formula is undefined and the calculator will show an error.

How do you calculate Coulomb's law?

Multiply the two charges, divide by the square of the distance between them, then multiply by k = 8.9875 × 10⁹ N·m²/C². For example, two 1 µC charges 1 m apart give F = 8.9875 × 10⁹ × (1e-6 × 1e-6) / 1² ≈ 8.99 × 10⁻³ N.

Coulombs Law Calculator

Name: Coulombs Law Calculator
Availability: InStock
Author: Nham Vu

Compute the electrostatic force between two point charges — or solve for charge or separation distance — using Coulomb's Law.

Coulomb's Law — F = k · q₁ · q₂ / r²

Select what to solve for, then enter the three known values.

Solve for

Charge 1 (q₁)

Charge 2 (q₂)

Separation Distance (r)

Result

Enter values and click Calculate

Formula Reference

Coulomb's Law F = k·q₁·q₂/r²

Solve for q₁ q₁ = F·r²/(k·q₂)

Solve for q₂ q₂ = F·r²/(k·q₁)

Solve for r r = √(k·q₁·q₂/F)

Coulomb constant (k) 8.9875 × 10⁹ N·m²/C²

Summary

Compute the electrostatic force between two point charges — or solve for charge or separation distance — using Coulomb's Law.

How it works

Select which quantity you want to solve for: Force, Charge 1, Charge 2, or Distance.
Enter the three known values with their units.
Click Calculate to apply F = k·q₁·q₂/r².
Read the result in multiple convenient units (N, µN, mN, or kN for force).
Use the Reset button to start a new calculation.

Use cases

Solve electrostatics homework and exam problems.
Verify hand calculations for physics lab reports.
Explore how force changes with distance or charge magnitude.
Teach or study the inverse-square law relationship.
Quickly convert between charge units (C, µC, nC, pC).
Find the separation needed to achieve a target force.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu