This voltage drop calculator estimates drop as current times resistance times an optional length factor. Enter amperes, ohms, and a length factor when you already know resistance for the run. Read the voltage drop result and compare it with the supply voltage for a simple percent check.
Electricians and DIY 12V planners often need a quick Ohms law style estimate. This tool is that simple model. It is not a substitute for code compliant wire sizing charts.
Formula used on this page
Default fields: 10 A, 0.5 Ω, length factor 1. Drop = 10 × 0.5 × 1 = 5 V.
If your length factor represents scaled ohms per unit length already baked into R, leave length at 1. If R is ohms per unit length, put the one way distance in the length field only when that matches how you defined R.
| Input | Value |
|---|---|
| Current | 10 A |
| Resistance | 0.5 Ω |
| Length factor | 1 |
| Voltage drop | 5 V |
How to use it responsibly
- Measure or look up conductor resistance for the wire you actually use.
- Enter load current in amperes.
- Keep units consistent for the length factor.
- Compare drop with source voltage. On a 12 V system, a 5 V drop is severe.
Circuit type and distance conventions
Many DC charts use round trip length because current travels out and back. If your resistance already covers the round trip, do not double count. State your assumption next to the number.
Copper and aluminum differ. Temperature changes resistance. This calculator does not switch materials for you.
Model limitations
There is no built in AWG table, conduit fill, or AC power factor model. For house wiring, follow local electrical code and a qualified electrician. Treat results as educational estimates.
Common mistakes
- Using one way feet while R already includes round trip ohms
- Ignoring that percent drop matters relative to supply voltage
- Treating this as approval to undersize a conductor
When to redesign the run
If percent drop is high, shorten the run, raise conductor size, or lower current. Do not ignore repeated brownouts on sensitive loads.
Automotive and solar DC
12 V and 48 V systems feel voltage drop quickly because the supply voltage is low. Percent drop matters more than the raw volt number alone.
Documentation habit
Write the assumed R, length convention, and load current next to the result so a later change does not reuse the wrong ohms.
Practical planning tips
Write down the inputs you used so you can repeat the estimate later. Small changes in rate, depth, hours, or price can move the result more than people expect.
When you compare two options, change one variable at a time. That keeps the comparison honest and easy to explain to a partner, client, or lender.
Units and rounding
Keep units consistent across every field. Mixing inches with feet, litres with gallons, or monthly figures with annual figures is the most common source of wrong answers.
Round only at the end for ordering or payments. Early rounding in each step can stack into a surprising gap versus the live calculator.
What this estimate does not include
Taxes, delivery fees, labor, penalties, and one time service charges are often outside the core formula. Add those on paper when you build a real budget.
Local rules, code requirements, and lender overlays can still change the final decision even when the math is correct.
How to double check the result
Recalculate once with a pencil using the same formula shown on this page. If your manual result is close to the tool, your inputs are probably consistent.
If the two answers diverge, check the field labels again before you trust either number for spending or filing.
When to get a second opinion
Large purchases, structural work, medical or legal questions, and loan contracts deserve a qualified human review. Use this page to prepare better questions, not to replace that review.
Save a screenshot or note of the inputs and result date. That record helps when you talk with a contractor, accountant, or lender later.
For related unit and percent checks while you plan materials, try the unit converter or the percentage calculator.