About this tool
Ohm's Law (1827) and the related Power Law are the foundation of electrical circuit analysis. Voltage drives current through resistance; the power dissipated is voltage × current. The calculator lets you solve for any one variable given any two others.
How it works
Enter the values you know and leave unknowns as 0. The calculator detects which pair you've provided and computes the rest using V=IR and P=VI.
Ohm's law - the foundation
V = I x R | I = V / R | R = V / I | P = V x I = I^2 x R = V^2 / R
Where V = volts, I = amperes, R = ohms, P = watts. Memorize this triangle and you can solve 80% of basic circuits.
Resistor color code (4-band)
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | 10^0 = 1 | - |
| Brown | 1 | 10^1 = 10 | ±1% (1B) |
| Red | 2 | 10^2 = 100 | ±2% (2B) |
| Orange | 3 | 10^3 = 1k | - |
| Yellow | 4 | 10^4 = 10k | - |
| Green | 5 | 10^5 = 100k | ±0.5% (5B) |
| Blue | 6 | 10^6 = 1M | ±0.25% |
| Violet | 7 | 10^7 = 10M | ±0.1% |
| Grey | 8 | 10^8 | - |
| White | 9 | 10^9 | - |
| Gold | - | 10^-1 | ±5% (4B) |
| Silver | - | 10^-2 | ±10% (4B) |
Read: first 2-3 bands are digits, next band is multiplier, last band is tolerance. Example: Brown-Black-Red-Gold = 1, 0, x100, ±5% = 1000Ω (1kΩ) ±5%.
Series vs parallel resistance
- Series: R_total = R1 + R2 + R3 + ...
- Parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ... (always less than the smallest resistor)
- Two parallel: R_total = (R1 x R2) / (R1 + R2)
LED current limiting
LEDs are current-driven devices. Always use a current-limiting resistor:
R = (V_supply - V_forward) / I_forward
Typical forward voltages and currents:
| LED color | V_forward | I_forward (typical) |
|---|---|---|
| Red, Orange, Yellow | 1.8-2.2V | 20 mA |
| Green (standard) | 2.0-2.2V | 20 mA |
| Green (pure) | 3.2-3.8V | 20 mA |
| Blue, White | 3.0-3.6V | 20 mA |
| UV | 3.6-4.4V | 20 mA |
| Infrared | 1.2-1.6V | 20-50 mA |
| High-power 1W | 3.0-3.6V | 350 mA |
Example: red LED on 5V supply: R = (5 - 2) / 0.020 = 150Ω. Use 180Ω or 220Ω for safety margin.
555 timer modes
- Monostable: triggered pulse with fixed duration. T = 1.1 x R x C. Used for debouncing, single-shot timers.
- Astable: continuous oscillation. Frequency = 1.44 / ((R1 + 2 x R2) x C). Used for square wave generators, LED blinkers, simple tone makers.
- Bistable: SR latch. State stays until reset.
The formula explained
This calculator uses the following formula:
V = I·R · P = V·I = I²·R = V²/R · enter any two, leave others as 0
The reason this formula works is rooted in the underlying physics, finance, or biology of the problem. Behind every calculator is a published, peer-reviewed equation or a widely accepted convention. We do not invent formulas; we apply standard ones from textbooks, government tables, professional bodies, and academic literature.
If you are curious about the math, the simplest way to verify is to plug in two known numbers and compare against a known result. The calculator should match published examples to within rounding precision.
Frequently asked questions
How does Ohm's Law apply to AC?
It works the same for resistive loads. With reactive (inductive/capacitive) loads, replace R with impedance Z and use complex math.
Why does current cause heating?
Power dissipation = I²R. Higher current through a resistor produces heat (which is why thick wires reduce loss).
USB power calculation?
USB-C PD: 20V × 5A = 100W. Standard USB: 5V × 0.5A = 2.5W. USB 3: 5V × 0.9A = 4.5W.
Battery capacity?
Watt-hours (Wh) = volts × amp-hours. A 12V 100Ah battery has 1200 Wh of energy.
Why do resistors burn out?
Exceeded power rating. P = V^2 / R or I^2 x R. A 1/4 watt resistor at 12V across 10 ohms dissipates 14.4 watts - 57x its rating. Always check power dissipation, especially for current-limiting and bleeder resistors.
Can I parallel batteries to get more capacity?
Yes for the same chemistry and voltage. Two 1500 mAh AA batteries in parallel give 3000 mAh at 1.5V. NEVER parallel batteries of different chemistry or charge state - they'll discharge into each other.
How do I read SMD resistor codes?
3-digit: first two are digits, third is multiplier (number of zeros). 472 = 47 x 10^2 = 4700 ohms = 4.7K. 4-digit: first three are digits, fourth is multiplier. 4701 = 470 x 10^1 = 4700 ohms. The R marks the decimal point: 4R7 = 4.7 ohms.
Is 3.3V or 5V better for hobby projects?
Modern microcontrollers (ESP32, RPi Pico, most STM32, ARM Cortex) run at 3.3V. Sensors and modules increasingly come at 3.3V. Use 3.3V for new builds unless you specifically need 5V (legacy Arduino shields, some motor drivers, USB).
What's the difference between AC and DC voltage ratings?
DC is constant; AC is peak voltage. A 'AC 12V' rating typically means RMS (Root Mean Square), so peak is 12 x sqrt(2) = 17V. Components rated for 12V DC may fail on 12V AC. Check spec sheets for AC vs DC rating columns.
How accurate is the Ohm's Law Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Ohm's Law Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the Ohm's Law Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the Ohm's Law Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the Ohm's Law Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the Ohm's Law Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another ohm's law tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.