Ohm's law, resistor color codes and LED resistors, voltage drop and wire size, battery runtime, circuit load and running cost. Works offline. Nothing you type leaves this device.
Fill in any two and the other two fill themselves. Typing a third clears the one you entered longest ago. Shorthand works: 4.7k, 4k7, 250m (milli), 2M2.
V = I × R and P = V × I, rearranged every way. This is exact for anything that behaves like a resistor: heaters, incandescent bulbs, toaster elements, the wire itself. Motors, LED drivers, chargers and electronics do not follow Ohm's law from the outside, so use the watts and volts on their label and solve for amps. On AC, a motor's watts are volts × amps × its power factor (often 0.7 to 0.9), so its amps run higher than watts ÷ volts.
🎚️ Resistor color code either way
Pick the colors to read a resistor, or type a value to see its bands. Read from the end the bands crowd toward; the tolerance band (often gold) sits apart on the other end.
4k7 = 4.7 kΩ, 1R5 = 1.5 Ω, R47 = 0.47 Ω
💡 LED resistor
The resistor that sets an LED's current. Forward voltage and current are on the LED's datasheet; the color presets are typical.
3.3591224
251020
➕ Series and parallel
Totals for a list of resistors, and the closest standard parts to a value you need.
Separate with commas or spaces.
Color bands follow IEC 60062: two or three digits, then a multiplier (gold ×0.1, silver ×0.01), then tolerance, and on 6-band parts a temperature coefficient. A resistor that reads as a non-standard value from one end is usually being read backwards. LED resistor: R = (supply − LED volts) ÷ current. The calculator rounds up to the next E12 value so the LED gets slightly less current, never more, and suggests a resistor rated for at least twice the watts it burns. Never wire LEDs in parallel on one resistor; give each string its own. Series resistances add; parallel is 1 ÷ (1/R1 + 1/R2 + …), always less than the smallest one.
📉 Voltage drop and wire size NEC tables
How much voltage a wire run loses, and the smallest wire that keeps the loss under your target. Distance is one way, from the panel or battery to the load.
122448120208240480
Watts ÷ volts if you only know watts.
Every wire size for this run
Resistance is NEC Chapter 9, Table 8 (stranded, uncoated, at 75 °C), so this is the drop on a warm wire. On AC, conductors bigger than about 2 AWG in steel conduit add a little reactance on top (NEC Table 9), so treat large-feeder numbers as slightly optimistic. 3% for a branch circuit and 5% overall are the NEC's recommendations (informational notes), not requirements; for 12 V and 24 V systems, solar and RV builders usually hold 3% or less because a volt matters so much more. Ampacity is NEC Table 310.16 (no more than three current-carrying conductors together, 30 °C ambient); bundling, attics, heat and terminal ratings lower it, and 14, 12 and 10 AWG copper are capped at 15, 20 and 30 A breakers no matter what the table says. A planning tool, not a substitute for your local code or an electrician.
🔋 Battery and power-station runtime
How long a battery runs a load, how long it takes to recharge, and what size you would need for a target runtime. Portable power stations list their capacity in Wh on the label.
3.71212.82425.64851.2
Needed for Ah or mAh, and for the amps drawn.
Wh = Ah × volts, which is why a 100 Ah 12 V battery (1,280 Wh at a lithium 12.8 V) holds about the same energy as a 1,280 Wh power station. Lead-acid batteries last far longer if you only use the top half, so they count as 50% usable; lithium is good to about 90%. An inverter burns about 10% turning DC into AC, and more at very light loads. Recharge assumes about 90% of the charger's watts reach the battery; solar panels rarely make their full rated watts, so plan on 60–80% of the panel rating on a good day. Real batteries deliver a bit less in the cold and at very high draw.
🔌 What fits on a circuit
Add what will run at the same time on one breaker. Tick continuous for anything that runs 3 hours or more (heaters, chargers, a window AC).
What
Watts
Continuous
Quick add (typical watts, check the nameplate):
The 80% rule. The NEC sizes a breaker at 125% of its continuous load, which is the same as saying continuous loads should stay under 80% of the breaker: 12 A on a 15 A circuit, 16 A on a 20 A. Short loads (a microwave, a hair dryer, a saw) can use the full rating. Motors and compressors (fridges, AC units, pumps, air compressors) pull 3 to 6 times their running amps for a moment at start-up, so a circuit that adds up fine can still trip when one kicks on. Nameplate watts are a maximum; most things draw less most of the time.
💵 What it costs to run
Energy use and cost for anything with a watt rating. Your rate is on your power bill as cents or dollars per kWh (add the delivery charges for the true number).
14824
Type 17¢ as 0.17.
kWh = watts × hours ÷ 1,000. A 1,500 W space heater running 8 hours uses 12 kWh a day. Things with a thermostat or compressor (fridges, heaters on a thermostat, AC) only draw full power part of the time, so their real use is often a third to a half of the nameplate math; a plug-in watt meter tells you the truth. A month here is 365 ÷ 12 days.
Good to know gotchas
Distance is one way. Current goes out and comes back, so a 100 ft run is 200 ft of wire; the voltage-drop math already doubles it. Entering the round trip doubles the drop twice.
Low voltage needs much bigger wire. The same 1,200 W at 12 V is 100 A instead of 10 A at 120 V, and a 1-volt loss is 8% of 12 V but under 1% of 120 V. That is why RV and solar battery cables are 2/0 and 4/0.
Wire that is big enough for the amps can still be too small for the distance. 12 AWG is fine for 20 A on a 120 V circuit, but on a 150 ft run it drops almost 10%, and motors and chargers struggle on low voltage.
Ah is not energy, Wh is. A 100 Ah battery at 12 V holds a quarter of what a 100 Ah battery at 48 V holds. Compare batteries and power stations in Wh.
Breakers protect the wire, not the appliance. Never put a bigger breaker on a circuit because it keeps tripping; the wire size sets the breaker. Converting between wire gauge, mm² and diameter is on the unit converter.