PowerVoltEdge

Electrical formulas

The Ohm's law and power wheel shows all 12 ways to find voltage, current, resistance and power. Below it is every formula you'll need for Level 2 and on site, each linked to its calculator or lesson.

  • V – Voltage
  • I – Current
  • R, Z – Resistance, impedance
  • P – Power
  • E – Energy
  • Q – Charge
  • f – Frequency

The formula wheel

The middle shows what you want to find. The outer ring shows the three ways to find it. Tap a quarter, or a letter below.

I want to find

Ohm's law and power formula wheel. Voltage V equals I times R, P divided by I, or the square root of P times R. Current I equals V divided by R, P divided by V, or the square root of P divided by R. Resistance R equals V divided by I, V squared divided by P, or P divided by I squared. Power P equals V times I, I squared times R, or V squared divided by R. √(P × R)P ÷ II × RVVolts V ÷ IV² ÷ PP ÷ I²ROhms V² ÷ RI² × RV × IPWatts V ÷ RP ÷ V√(P ÷ R)IAmps

Pick a letter to see every way to find it.

I know two values

Tap the two you have and you'll see how to get the other two.

Tap two letters.

Want the numbers worked out? Use the Ohm's law calculator: enter any two values and it gives the other two.

Formula triangles

Cover the one you want to find. What's left tells you whether to multiply (side by side) or divide (one over the other).

Formula triangle: V (voltage) on top, I (current) and R (resistance) underneath. V = I × R, I = V ÷ R, R = V ÷ I. VIR V = I × R Formula triangle: P (power) on top, V (voltage) and I (current) underneath. P = V × I, V = P ÷ I, I = P ÷ V. PVI P = V × I Formula triangle: E (energy) on top, P (power) and t (time) underneath. E = P × t, P = E ÷ t, t = E ÷ P. EPt E = P × t Formula triangle: Q (charge) on top, I (current) and t (time) underneath. Q = I × t, I = Q ÷ t, t = Q ÷ I. QIt Q = I × t

Formula sheet

Ohm's law and power

FormulaWhat it's for
V = I × RVoltage from current and resistance. Open
P = V × IPower from voltage and current. Open
P = I² × RPower from current and resistance. Open
P = V² ÷ RPower from voltage and resistance. Open

Energy and charge

FormulaWhat it's for
E = P × tEnergy: kW × hours gives kWh. Open
Q = I × tCharge: amps × seconds gives coulombs. Open

Series and parallel

FormulaWhat it's for
RT = R1 + R2 + R3Total resistance in series. Open
1 ÷ RT = 1 ÷ R1 + 1 ÷ R2 + 1 ÷ R3Total resistance in parallel. Open
RT = (R1 × R2) ÷ (R1 + R2)Two resistors in parallel: product over sum. Open

Current from a load

FormulaWhat it's for
I = P ÷ (V × pf)Single phase, with power factor pf. Open
I = P ÷ (√3 × VL × pf)Three phase, VL = 400 V line to line. Open

Cable selection and testing

FormulaWhat it's for
Ib ≤ In ≤ IzDesign current ≤ device rating ≤ cable capacity. Open
It ≥ In ÷ (Ca × Cg × Ci × Cc)Tabulated rating needed after correction factors. Open
Vd = (mV/A/m × Ib × L) ÷ 1000Voltage drop, L in metres. Open
Zs = Ze + (R1 + R2)Earth fault loop impedance of a circuit. Open
Zs = (Uo × Cmin) ÷ IaMaximum Zs for a circuit-breaker (Uo 230 V, Cmin 0.95). Open
S = √(I² × t) ÷ kMinimum CPC size (adiabatic equation). Open

AC

FormulaWhat it's for
Z = V ÷ IImpedance: the total opposition to current in an AC circuit, in ohms. Open
f = 1 ÷ TFrequency from the time for one cycle, T in seconds. UK mains is 50 Hz. Open

Maths

FormulaWhat it's for
Area = π × d² ÷ 4Cross-sectional area from a conductor's diameter. Open
c² = a² + b²Pythagoras: the longest side of a right-angled triangle. Open
Efficiency = output ÷ input × 100 %How much input power does useful work. Open

Use base units (V, A, Ω, W) in these formulas unless a line says otherwise. Check against BS 7671 and the On-Site Guide before relying on a result.