Ohm’s Law Calculator | Celtic Engineering Solutions
Celtic Engineering Toolbox
Ohm’s Law Calculator (V, I, R, P)
Fast, practical Ohm’s Law calculator used in real client projects at
Celtic Engineering Solutions LLC. Enter any two of voltage, current,
and resistance; we’ll compute the third and the power.
Ohm’s Law Calculator
Fill in any two values below and leave
one box blank. The calculator will solve for the
missing value and show power.
Quick presets
Core formulas and unit reminders
This calculator uses the basic Ohm’s Law and power relationships:
V = I × R (voltage)
I = V ÷ R (current)
R = V ÷ I (resistance)
P = V × I (power in watts)
Units used on this page:
Voltage in volts: V
Current in amps: A (0.02 A = 20 mA)
Resistance in ohms: Ω (1 kΩ = 1,000 Ω)
Power in watts: W
Scope.
This tool is designed for low-voltage electronics (roughly 0–1000 V).
For mains or high-voltage work, always follow relevant safety standards,
clearance/creepage rules, and derating guidelines.
Quick design checks before you order parts
Use this checklist as a fast sanity check after you run the numbers.
Resistor power.
Compare the calculated power (P) against the resistor’s
rated power (¼ W, ½ W, 1 W, etc.). Use at least a 2× margin.
Source limits.
Verify that the current is within the limits of the source
(microcontroller pin, regulator, power supply).
Tolerance and variation.
Real resistors have tolerance (±1%, ±5%). Consider both the highest
and lowest possible current with those extremes.
Temperature.
High power in a small package causes heating, drift, and reliability
issues. When in doubt, move up in power rating and give the part
room to breathe.
Safety-critical paths.
For anything involving mains, high energy storage, or safety-related
circuits, treat this tool as a starting point, not the final word.
Examples from Celtic Engineering Solutions
These are simplified versions of real situations where a quick Ohm’s
Law check kept prototypes safe and predictable.
Example A
Limiting LED current on a 5 V microcontroller board
A status LED needed a forward current around 15–20 mA from a
5 V logic rail. The LED’s forward voltage was roughly
2 V.
Using V = I × R:
• V across the resistor ≈ 5 V − 2 V = 3 V
• Target current ≈ 0.015–0.02 A
• R ≈ 3 V ÷ 0.02 A = 150 Ω
The design used 220 Ω to be gentle on the LED and the MCU
pin, with calculated power well under 0.25 W.
Example B
Choosing a sense resistor for a current-limited driver
A driver relied on a small sense resistor to limit current. The
control IC expected about 0.5 V across the sense resistor at
the target current of 100 mA.
• V across sense resistor = 0.5 V
• I = 0.1 A
• R = 0.5 V ÷ 0.1 A = 5 Ω
• Power = 0.5 V × 0.1 A = 0.05 W
A ¼ W 5 Ω resistor provided comfortable margin, and
the driver behaved as expected on the bench.
Example C
Sanity-checking a box build at 24 V
A small enclosure used a 24 V supply feeding several
resistive loads. A quick check confirmed currents and power were
within connector and wiring limits.
For one branch:
• V = 24 V, R = 4.7 kΩ
• I ≈ 24 ÷ 4700 ≈ 5.1 mA
• P ≈ 24 × 0.0051 ≈ 0.12 W
Standard ¼ W resistors were sufficient, and heat rise inside
the enclosure stayed modest.
Example D
Protecting an input from over-current
A board needed to accept a 12 V signal while feeding an
isolated input stage that liked only a few milliamps.
Target current was set to 3 mA for robustness.
• V = 12 V, I = 0.003 A
• R = 12 ÷ 0.003 ≈ 4 kΩ
• Chosen value: 4.3 kΩ
• P ≈ 12 × 0.003 ≈ 0.036 W
Even a ⅛ W part would have worked; a ¼ W part was
used for added headroom.
Not sure if your numbers are safe?
If your calculation involves mains voltage, motor loads, or anything
safety-critical, treat this as a starting point. Send a short note with
your values and constraints, and we’ll help you confirm a safe design
direction.
