Volts to Amps Calculator
Convert volts to amps by either route — Ohm's law from a resistance, or power from a load in watts.
Updated
Ohm's law: current is voltage divided by resistance.
Current
2.00 Aof current
120 V across 60.00 ohms
- Amps
- 2.000 A
- Power
- 240.0 W
- Resistance
- 60.00 ohms
- Breaker at 80 percent
- 2.5 A
Resistance route: I = V / R. The power figure is derived from the answer, not from a rating.
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In short
How do you convert volts to amps?
You cannot, from voltage alone. Current needs a second quantity: with a resistance, Ohm’s law gives I = V / R, so 120 V across 60 ohms is 2 A. With a load rating instead, I = P / V, so a 1,500 W appliance on 120 V draws 12.5 A.
Two routes, two different answers on the same supply. Which one is right depends on what you measured.
How to use the volts to amps calculator
Enter the voltage, say which other quantity you know, and read the current. The reason the page asks rather than assumes is that voltage on its own genuinely does not determine current, and a converter that pretends otherwise has quietly invented a number.
Voltage is the push and current is the flow. How much flows depends on what is in the way, and that is either stated as a resistance or implied by a load rating. Give the same 120 volt supply a heating element of 9.6 ohms and it draws 12.5 amps; give it a 1,200 ohm resistor and it draws a tenth of an amp. Same voltage, a hundred and twenty-five times the current.
I = V / R
Ohm law route
when you have resistance
I = P / V
Power route
when you have a wattage
12.5 A
A 1,500 W load at 120 V
the standard heater
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Which route to use is decided by what you actually have in front of you. A meter across a component gives ohms, so Ohm’s law is the one. A nameplate on an appliance gives watts, so the power relationship is the one. The two are consistent with each other, and the tool shows the quantity you did not supply so you can check.
The breaker figure in the results is there because it is the question underneath most of these lookups. A circuit should carry no more than eighty percent of its rating continuously, so a 12.5 amp load wants a breaker of at least 15.6 amps, which in practice means a 20 amp circuit rather than a 15.
- 60 W bulb
- 0.5 A · 240 ohms
- 600 W microwave
- 5 A · 24 ohms
- 1,200 W kettle
- 10 A · 12 ohms
- 1,500 W heater
- 12.5 A · 9.6 ohms
- 1,800 W, a full 15 A circuit
- 15 A · 8 ohms
The resistance column falls as the load rises, which is the part people find counterintuitive. A more powerful appliance presents LESS resistance, because resistance is what limits the flow and a big load is one that limits it least.
Turn that current into a bill
Amps to kWh takes the current, the voltage and the running time and produces the energy used, which is the figure a bill is actually built from.
Open amps to kWh →Current drawn on common supply voltages, worked from a resistance under Ohm law and from a load rating under the power relationship.
| Supply | Resistance | Current (I = V/R) | Same current from |
|---|---|---|---|
| 12 V | 4 ohms | 3.00 A | 36 W |
| 12 V | 2.4 ohms | 5.00 A | 60 W |
| 120 V | 1,200 ohms | 0.10 A | 12 W |
| 120 V | 240 ohms | 0.50 A | 60 W |
| 120 V | 24 ohms | 5.00 A | 600 W |
| 120 V | 12 ohms | 10.00 A | 1,200 W |
| 120 V | 9.6 ohms | 12.50 A | 1,500 W |
| 120 V | 8 ohms | 15.00 A | 1,800 W |
| 240 V | 60 ohms | 4.00 A | 960 W |
| 240 V | 19.2 ohms | 12.50 A | 3,000 W |
Two routes, and when they disagree
The two relationships are not alternatives to each other so much as two views of the same circuit. Ohm’s law describes what the component does; the power equation describes what the appliance is rated to consume. On a purely resistive load they agree exactly, and the tool derives whichever you did not enter.
- Nameplate
- 1,500 W at 120 V
- Power route
- 12.5 A
- Implied resistance
- 9.6 ohms
- Meter across the cold element
- reads lower
- Because resistance rises with temperature
- in a metal element
- Agreement
- only once it is hot
A cold heating element measures noticeably less resistance than its running value, so the inrush current on switch-on is higher than the steady figure. That is normal, it is brief, and it is why breakers are designed to tolerate a short overload rather than trip on it.
Where the two genuinely disagree, something has been misread. A nameplate wattage that implies a resistance nowhere near the measured one usually means the appliance is not purely resistive, or that the rating is a maximum rather than a continuous figure.
What the current figure is actually for
Current is the quantity that decides safety. Wire gauge, breaker size and connector rating are all specified in amps, because heat in a conductor rises with the square of the current and has nothing to do with the voltage behind it.
Do
- Use the resistance route when you have measured a component
- Use the wattage route when you have a nameplate rating
- Apply the eighty percent rule to any continuous load
- Check the cold resistance of a heating element separately
Don't
- Try to get current from voltage alone
- Apply these formulas to a motor or transformer
- Load a circuit to its full rating continuously
- Assume a higher voltage means a higher current
That last one is worth stating because it is exactly backwards. For a fixed power, doubling the voltage halves the current, which is why long-distance transmission runs at hundreds of thousands of volts and why the same appliance draws half as many amps on a 240 volt supply as on a 120 volt one.
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The formula, worked line by line
Two formulas, and the page will not run either until you say which one applies. That is the honest behaviour: there is no third formula that gets current from voltage alone.
“Voltage is the push. Current is the flow. Nothing about the push tells you the flow until you know what is in the way.”
Ohm published the relationship in 1827 and it is still the first thing anyone learns about circuits, precisely because it is the thing that connects the two quantities people can measure.
Ohm law: amps = volts / ohms
Power: amps = watts / volts
120 V / 60 ohms = 2 A
1,500 W / 120 V = 12.5 A
continuous breaker = amps / 0.8- Heater
- 1,500 W at 120 V
- Current
- 12.5 A
- Continuous load rule
- 80 percent of breaker
- Minimum breaker
- 15.6 A
- Next standard size
- 20 A
- Verdict
- a 15 A circuit is not enough
A 15 amp breaker is rated for 12 amps continuously, so a 12.5 amp heater sits just over the line. It will often run without tripping and that is the problem: a circuit loaded to its limit for hours is how insulation degrades quietly over years.
Why higher voltage means lower current
Power is voltage times current, so for a fixed power the two trade off exactly. The same 3,000 watt appliance draws 25 amps at 120 volts and 12.5 amps at 240. Halving the current quarters the heating loss in the wire, which is why heavy appliances and whole countries run at the higher figure.
What decides the wire, not the voltage
- Wire gauge
- chosen by amps
- Breaker size
- chosen by amps
- Connector rating
- chosen by amps
- Insulation thickness
- chosen by volts
- Heating in a conductor
- rises with amps squared
Voltage decides how much insulation a cable needs; current decides how thick the copper inside it has to be. That is why a thin high-voltage cable and a thick low-voltage one can carry the same power.
The line to carry away: if a question about current does not come with either a resistance or a wattage, the question is incomplete rather than difficult.
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Questions people ask
How many amps is 120 volts?
The question has no answer on its own. Voltage is the push and current depends on what resists it. With a resistance you use Ohm’s law, so 120 V across 60 ohms is 2 A. With an appliance rating you divide watts by volts, so 1,500 W at 120 V is 12.5 A.
What is Ohm law?
Current equals voltage divided by resistance, written I = V / R. It is the relationship connecting the three basic quantities of a circuit, published by Georg Ohm in 1827. Rearranged, it also gives voltage as current times resistance, and resistance as voltage divided by current.
Which route should I use, ohms or watts?
Whichever quantity you actually have. A meter across a component gives you resistance, so use Ohm’s law. An appliance nameplate gives you watts, so divide by the voltage. On a purely resistive load the two agree, and this tool shows the one you did not enter so you can check them against each other.
Does this work for motors?
Not reliably. Alternating current through a coil produces reactance as well as resistance, so a motor draws more current than the simple division suggests for the same real power. The ratio between the two is the power factor, and it needs its own calculation rather than this one.
Why does a bigger appliance have less resistance?
Because resistance is what limits the flow. A high-power load is one that limits current least, so it presents a low resistance: a 1,500 W heater on 120 V is 9.6 ohms, while a 60 W bulb is 240 ohms. The counterintuitive part is that the small appliance is the one resisting hardest.
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