Free calculators for automation and electrical work. Every answer shows the formula and the method, so you can check it and learn it. Below them is the Ladder Lab, a PLC simulator that runs in your browser.
Engineering tools
Ohm's law & power
V, I, R and P from any two values
Formula and notes
V = I × R P = V × I = I² × R = V² / R
Fill in any two values and leave the others empty. Example: a 24 V coil with 120 Ω resistance draws 0.2 A and dissipates 4.8 W.
4–20 mA scaling
Signal to engineering units and back
Formula and notes
EU = LO + (mA − 4) / 16 × (HI − LO) mA = 4 + 16 × (EU − LO) / (HI − LO)
Example: a 0–10 bar transmitter reading 12 mA is at 5 bar. Below 3.8 mA usually means a broken wire (NAMUR NE43).
Modbus address converter
40001-style address to register and offset
Formula and notes
0xxxx coil (FC01/05/15) · 1xxxx discrete input (FC02) · 3xxxx input register (FC04) · 4xxxx holding register (FC03/06/16)
The first digit is the register type, the rest is the address. 40001 is holding register offset 0 in most devices, sent as function code 03.
PLC timer preset
Seconds to K value for any time base
Formula and notes
K = time (ms) / time base (ms)
Example: 5 s on a 100 ms timer is K50. Siemens S7 TON timers take a time value directly (e.g. T#5S).
Cable voltage drop
Single or three-phase, copper or aluminium
Formula and notes
ΔV = 2 × L × I × ρ / A (single-phase) ΔV = √3 × L × I × ρ / A (three-phase) ρ: Cu 0.0175, Al 0.028 Ω·mm²/m
A quick resistive estimate at 20 °C. Many installations aim for under 3–5% drop; always check your local wiring regulations and the cable maker's tables.
Motor full-load current
kW, voltage, PF and efficiency to amps
Formula and notes
I = P × 1000 / (√3 × V × PF × η)
An estimate for three-phase induction motors. Use the nameplate current for overload and VFD sizing when you have it.
LED series resistor
For Arduino, ESP32 and indicator circuits
Formula and notes
R = (Vs − Vf) / I
Typical forward voltage: red 1.8–2.2 V, green/blue/white about 3 V. Pick the next standard resistor value above the result.
NPN / PNP sensor wiring simulator
See which sensor works with which PLC input, and why
Formula and notes
PNP → sinking input (COM = 0 V) NPN → sourcing input (COM = +24 V)
Current must flow in a loop: from +24 V, through the sensor and the PLC input LED, back to 0 V. A PNP sensor connects its output to +24 V, so the input must return to 0 V. An NPN sensor connects its output to 0 V, so the input must be fed from +24 V. Wire colours follow IEC 60947-5-2: brown +V, blue 0 V, black output.
Simulator
Ladder Lab
Pick a circuit, press the inputs and watch power flow through each rung. The instruction list underneath is the same program in text form.
Program listing (IL)
LD X0
OR Y0
ANI X1
OUT Y0