Ohm's Law Calculator
Calculate voltage, current, resistance and power from one selected pair of known electrical quantities.
Use the Ohm's Law Calculator
Calculate voltage, current, resistance and power from one selected pair of known electrical quantities.
Calculations happen locally in your browser. ToolLott does not send these inputs to a server for this tool.
Preliminary engineering calculation only. It does not replace applicable codes, manufacturer data, licensed electrical or mechanical design, protection studies, derating, fault checks, site conditions or professional verification.
How to use it
Replace the worked-example values with the actual project inputs. Review the intermediate quantities and assumptions before using the result elsewhere.
How the Ohm's Law Calculator works
Calculate voltage, current, resistance and power from one selected pair of known electrical quantities. The methodology below exposes the production equation, a QA-verified worked example and the boundary between this screen and detailed design.
Calculation breakdown
ToolLott will explain the current inputs and displayed result here.
Ohm's law and electrical power
The production engine evaluates V = I R; P = V I. It begins by use the selected pair of known electrical quantities., then rearrange V = I R to calculate the missing voltage, current or resistance., and finally calculate electrical power from P = V I. This keeps the calculation auditable instead of hiding design assumptions behind a single number.
V = I R; P = V IToolLott evaluates the relationship using the entered units and full numeric precision before display rounding.
| Symbol / input | Meaning | Unit |
|---|---|---|
mode | Known values | user input |
voltage | Voltage (V) | user input |
current | Current (A) | user input |
resistance | Resistance (ohm) | user input |
Step-by-step method
- Use the selected pair of known electrical quantities.
- Rearrange V = I R to calculate the missing voltage, current or resistance.
- Calculate electrical power from P = V I.
Worked example
Ethan, a project engineer, measured 24 V across a 12 Ω test load and wants to check expected current before comparing it with the meter reading.
Example inputs
- Known values: vr
- Voltage (V): 24
- Current (A): 2
- Resistance (ohm): 12
Calculation / processing
With V = 24 V and R = 12 ohm, I = 24 / 12 = 2 A.Power = 24 x 2 = 48 W.The page therefore reports 2 A, 24 V, 12 ohm and 48 W.
Use the result as a screening or quantity-planning value and compare it with the project criteria, manufacturer data and applicable engineering requirements before making a design or procurement decision.
Assumptions
- The entered values are representative of the condition being screened and use the units shown on the page.
- The equation models only the stated relationship; omitted system effects are not silently estimated.
Limitations
- This page is not engineering certification and does not replace applicable standards, design loads, manufacturer data, site investigation or competent professional review.
- Results can change materially when real geometry, losses, transients, material properties, duty cycles, safety factors or regulatory criteria differ from the simplified inputs.
Common questions
Is this a final engineering design?
No. It is a transparent screening calculation using the entered assumptions and the stated equation.
Why does the page show assumptions and limitations?
Engineering formulas are only valid within their modelling assumptions; exposing them helps users decide what additional design checks are required.
Should I use the rounded displayed value in later design work?
Use suitable calculation precision and the governing project/standard requirements rather than relying on display rounding alone.
Methodology sources
Related ToolLott tools
A realistic way Ethan could use this tool
Ethan is a project engineer.
Ethan measured 24 V across a 12 Ω test load and wants to check expected current before comparing it with the meter reading.
Ethan runs the example with mode = “vr”; voltage = 24; current = 2; resistance = 12. Processing that specific set gives Current: 2 A - Voltage: 24 V, making the result reproducible instead of relying on a generic claim about what the tool should do.
For this scenario, Current: 2 A - Voltage: 24 V. Ethan can use that specific result to check expected current before comparing it with the meter reading, while keeping the stated assumptions separate from the calculation itself. The page also keeps this limitation explicit: Preliminary screening or quantity calculation only.
Fictional scenario using realistic example data. For Ready tools, the worked result is tied to the tested example shown in the tool. Replace the figures with your own inputs and independently verify important professional, financial, legal, health or safety decisions.
What this calculator is for
Use it to calculate voltage, current, resistance and power from one selected pair of known electrical quantities.
It sits within ToolLott’s Electrical collection, where you can also estimate battery banks, cable sizes, generator sizes, motor current, Ohm’s law values, solar systems, transformers and voltage drop.