Voltage Calculator
Ohm's law solver for any 2 of V, I, R, P, voltage divider calculator online (forward, reverse & loaded), LED resistor calculator for 5V and 12V supplies, and a DC wire voltage drop calculator — with mV to V to kV conversion on every field.
Ohm's Law Calculator — V, I, R, P Solver
Enter exactly two of the four quantities. This V IR P calculator — a V=IR calculator and power voltage current resistance calculator — finds the other two using V = I × R and P = V × I, covering all 6 combinations.
Voltage Divider Calculator
Three modes: forward (learn how to calculate voltage divider output from Vin, R1, R2), reverse design (target Vout at a chosen divider current), and loaded (voltage divider with load across R2).
LED Resistor Calculator
LED current limiting resistor calculator: R = (Vs − Vf) / If. This LED resistor calculator (5V, 12V, or any supply) snaps up to standard E12 resistor values and includes a resistor power rating calculator using the 2× safety rule.
Wire Voltage Drop Calculator
Simple DC voltage drop calculator for copper wire — an AWG voltage drop calculator: Vdrop = 2 × R × L × I (the ×2 covers the round trip). Pick an AWG size to see the drop and whether it stays within the acceptable voltage drop percentage. To see how to calculate voltage drop in wire, enter supply voltage, current, one-way length, and AWG size below.
Worked Examples
Ohm's law: 12 V across 100 Ω
Current I = V / R = 12 / 100 = 0.12 A; power P = V² / R = 144 / 100 = 1.44 W.
Divider: 12 V → 5 V at 1 mA
Rtotal = 12 / 0.001 = 12 kΩ; R2 = 12k × 5/12 = 5 kΩ; R1 = 7 kΩ. Output: exactly 5 V.
LED: 5 V supply, 2 V LED, 20 mA
R = (5 − 2) / 0.02 = 150 Ω (already E12); power 0.06 W → use a ⅛ W resistor (≥ 2×).
Methodology & Formulas
| Quantity | Formula | When to use |
|---|---|---|
| Voltage | V = I × R, V = P / I, V = √(P × R) | Any two of V, I, R, P known |
| Current | I = V / R, I = P / V, I = √(P / R) | Any two of V, I, R, P known |
| Resistance | R = V / I, R = V² / P, R = P / I² | Any two of V, I, R, P known |
| Power | P = V × I = I²R = V² / R | Any two of V, I, R, P known |
| Divider output | Vout = Vin × R2 / (R1 + R2) | Output taken across R2 to ground |
| Divider design | Rtotal = Vin / Itot; R2 = Rtotal × Vout / Vin | Choose Itot ≥ 10× expected load current |
| Loaded divider | Req = R2 ‖ RL; Vout = Vin × Req / (R1 + Req) | Load RL in parallel with R2 |
| LED resistor | R = (Vs − Vf) / If | Round up to next E12 value |
| Wire drop (DC) | Vdrop = 2 × R × L × I | Copper, one-way length L, round trip ×2 |
Copper resistance per kilometre at 20 °C used by the wire-drop mode: 8 AWG 2.061, 10 AWG 3.277, 12 AWG 5.211, 14 AWG 8.286, 16 AWG 13.17, 18 AWG 20.95, 20 AWG 33.31, 22 AWG 52.96 Ω/km. Standard resistor values (E12): 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 (×10ⁿ). Resistor power ratings: ⅛, ¼, ½, 1, 2, 5 W — always pick ≥ 2× the dissipated power.
Reference/educational tool. For mains wiring or anything above low-voltage DC, consult a licensed electrician and your local electrical code.
Scope Boundary — What This Tool Is (and Isn't)
This voltage calculator calculates voltages: Ohm's law (solve any 2 of V, I, R, P), voltage dividers (forward, reverse design, loaded), LED series resistors with E12 snap, and simple DC wire voltage drop.
- Unit conversion is the job of the voltage converter: a voltage unit converter (mV, V, kV, µV) with dB scales. The unit dropdowns here are just input conveniences. (Note: the voltage converter also hosts its own divider solver and NEC drop tab; use this calculator when you want the consolidated DC voltage workflow — Ohm's law, dividers, LED resistors, and wire drop on one page.)
- Related calculators by specialty: resistor dividers & LED helpers are also covered by the resistor voltage drop calculator and the series resistor calculator; the full Ohm's-law hub (V/A/Ω/F/H + resistor codes + ampacity) lives at the electrical unit converter. This page is the voltage-focused consolidation, not a replacement for those deep-dives.
- Wire sizing (which AWG to choose for a load) is covered by the 12V wire size calculator; deep multi-resistor networks by the series resistor calculator; resistor color codes and ampacity tables by the electrical unit converter.
- AC / three-phase power (P = √3·V·I·PF) is out of scope — this tool is DC resistive circuits only.
Frequently Asked Questions
How do I calculate voltage using Ohm's law?
Multiply current by resistance: V = I × R. For example, 0.12 A through 100 Ω gives 12 V. To calculate voltage from current and resistance, enter any two of V, I, R, P in the Ohm's law solver above and it computes the other two.
How to find voltage from power and resistance?
Use the square root PR voltage formula: V = √(P × R). Example: a 1.44 W load on 100 Ω sees √(1.44 × 100) = 12 V. The solver handles all six V/I/R/P combinations automatically.
What is the voltage divider formula?
Vout = Vin × R2 / (R1 + R2), where R2 is the resistor between the output node and ground. Example: 12 V with R1 = 7 kΩ and R2 = 5 kΩ gives exactly 5 V.
How do I design a voltage divider for a target output voltage?
Pick the total divider current Itot first (rule of thumb: ≥10× the load current), then Rtotal = Vin / Itot, R2 = Rtotal × Vout / Vin, R1 = Rtotal − R2. Example: 12 V → 5 V at 1 mA needs R1 = 7 kΩ and R2 = 5 kΩ. The reverse voltage divider calculator mode above does this and snaps to the nearest E12 values.
Why does my voltage divider output drop when I connect a load?
The load sits in parallel with R2, lowering the effective lower resistance, so Vout falls. Use the loaded voltage divider calculator mode to see the exact loaded vs unloaded voltage and the droop percentage. Keep divider current ≥10× the load current, or use a voltage regulator for real loads.
Can you power a circuit with a voltage divider?
Not reliably — any change in load current shifts the output voltage, and the divider wastes power continuously. Dividers are for reference voltages and signal scaling. In the voltage divider vs voltage regulator debate, the regulator always wins for powering circuits.
How to calculate LED resistor value?
Use R = (Vs − Vf) / If, where Vs is supply voltage, Vf is the LED forward voltage, and If is the desired current. To calculate LED resistor value for a typical case: 5 V supply, 2 V LED, 20 mA → (5 − 2) / 0.02 = 150 Ω. Round up to the next E12 value and pick a power rating ≥2× the dissipated power.
Should I round an LED resistor up or down?
Up. A larger resistor means slightly less LED current, which is safe; rounding down pushes more current than intended. The LED mode above is an E12 resistor calculator that always snaps up to the next standard value.
How to calculate voltage drop in a wire?
Use the voltage drop formula Vdrop = 2 × R × L × I: R is the wire resistance per unit length, L is the one-way length, I is the current, and the factor 2 accounts for the round trip (out and back). To calculate voltage drop in a wire run, enter supply voltage, current, length, and AWG size in the wire-drop mode above.
What voltage drop percentage is acceptable?
For low-voltage DC wiring (12/24 V systems), the acceptable voltage drop percentage is ≤3% (OK), 3–5% is marginal, and above 5% the wire is too thin or the run too long. Mains wiring follows local electrical codes — consult a licensed electrician.
What is the difference between this voltage calculator and the voltage converter?
This tool calculates voltages: Ohm's law, voltage dividers, LED resistors, and wire voltage drop. The megawatt to kWh calculator handles bulk energy-unit conversions (MWh, GWh, joules). Use the converter when you need unit conversion.
How do I convert mV to V or V to kV by hand?
Divide or multiply by 1,000: 1 V = 1,000 mV = 0.001 kV, so mV to V kV conversion is just shifting the decimal point three places. For batch or exotic units (µV, MV, dBmV), use a dedicated mV to V kV conversion calculator like the voltage converter tool.
How to choose resistor wattage?
Choose a resistor wattage of at least twice the actual dissipated power: P = V × I across the resistor. Standard ratings are ⅛, ¼, ½, 1, 2, and 5 W — the calculators above suggest a rating automatically using this 2× rule.
What is the difference between volts, amps, watts, ohms?
Volts (V) measure electric potential, amps (A) measure current flow, ohms (Ω) measure resistance, and watts (W) measure power. They are linked by V = I × R and P = V × I — the two formulas this entire voltage calculator is built on.
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