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Torque, Ohm's Law and battery runtime are the three conversions that come up most in a working bay, so they share one page here. Torque unit conversion, Ohm's Law, and battery amp-hour runtime — the everyday math that comes up on the bench.
How this calculator works
Torque conversion uses standard, exact SI conversion factors. Ohm's Law and power together describe how voltage, current, resistance, and power relate in any DC circuit — handy for checking wire gauge against a fuse rating, sizing a battery for an accessory, or diagnosing why something's drawing more current than expected. Battery runtime assumes a constant, steady load — real-world runtime is usually a bit less once you factor in battery age, temperature, and voltage sag under load.
Worked examples: one from each tab
Torque. 35 ft-lb into the other two units:
- 35 × 1.3558179 = 47.45 N·m
- 35 × 12 = 420 in-lb
Ohm's law. 2 A through 6 Ω: V = I × R = 12 V, and P = V × I = 24 W.
Battery. A 100 Ah battery at 80% usable, drawing 10 A: 100 × 0.80 = 80 Ah usable, ÷ 10 A = 8 hours — and see the caveat below, because eight hours is the ideal figure, not the one you will get.
Torque conversion reference
Exact conversions: 1 ft-lb = 1.3558179 N·m = 12 in-lb. The in-lb column is the one people get wrong, because a torque wrench calibrated in inch-pounds and one calibrated in foot-pounds are a factor of twelve apart and both are common in the same toolbox.
| ft-lb | N·m | in-lb |
|---|---|---|
| 10 | 13.6 | 120 |
| 15 | 20.3 | 180 |
| 20 | 27.1 | 240 |
| 25 | 33.9 | 300 |
| 35 | 47.5 | 420 |
| 50 | 67.8 | 600 |
| 75 | 101.7 | 900 |
| 100 | 135.6 | 1,200 |
| 150 | 203.4 | 1,800 |
Converting is exact arithmetic; choosing the number is not. Always use the figure the service manual gives for that specific fastener, in the condition it specifies — dry, oiled or with thread locker, since lubrication changes the clamping force a given torque produces. Torque-to-yield and torque-plus-angle fasteners cannot be set with a torque figure alone at all.
Ohm's law and power, all four ways
| To find | From V and I | From V and R | From I and R |
|---|---|---|---|
| Voltage (V) | — | — | V = I × R |
| Current (I) | — | I = V ÷ R | — |
| Resistance (R) | R = V ÷ I | — | — |
| Power (W) | P = V × I | P = V² ÷ R | P = I² × R |
These hold for direct current, which is what a vehicle's electrical system is. On alternating current, power also depends on power factor and the simple P = V × I stops being the whole story. Note P = I² × R: doubling the current through a connection quadruples the heat it generates, which is why a loose or corroded terminal on a high-current circuit is a fire risk rather than just a voltage-drop problem.
Watts, amps and how long a battery lasts
Appliances are labelled in watts and batteries in amp-hours, so the first step is always watts ÷ volts = amps. Runtime below is on 80 Ah usable — a 100 Ah battery at the 80% the calculator defaults to.
| Load | Amps at 12 V | Amps at 24 V | Runtime on 80 Ah usable |
|---|---|---|---|
| 30 W | 2.50 | 1.25 | 32.0 hr |
| 60 W | 5.00 | 2.50 | 16.0 hr |
| 100 W | 8.33 | 4.17 | 9.6 hr |
| 150 W | 12.50 | 6.25 | 6.4 hr |
| 300 W | 25.00 | 12.50 | 3.2 hr |
| 600 W | 50.00 | 25.00 | 1.6 hr |
| 1,000 W | 83.33 | 41.67 | 0.96 hr |
The 24 V column is why heavy trucks use 24 V for starting: the same power at double the voltage halves the current, and halving current cuts the heat in the cabling to a quarter. If the load runs through an inverter, add its losses — a typical inverter is 85–90% efficient, so a 1,000 W load draws nearer 95 A than 83 A from a 12 V battery.
Why the runtime figure is optimistic
- Peukert's effect. A battery's amp-hour rating is measured at a slow, standardised discharge. Pull harder and you get fewer amp-hours out of the same battery — the effect is significant on lead-acid and small on lithium.
- Depth of discharge is chemistry-specific. Flooded lead-acid is conventionally worked to about 50%; AGM tolerates more; LiFePO₄ is usually rated to 80% or beyond. The usable-capacity percentage on this page is where you tell it which world you are in.
- Cold cuts capacity sharply, and a battery in an unheated truck in winter is a smaller battery than the label says.
- Age is cumulative. Capacity falls over the battery's life, so a three-year-old 100 Ah battery is not a 100 Ah battery.
- Nothing here models the alternator. This is a discharge calculation for a battery on its own.
- Deep discharge is not free. Taking a lead-acid battery below what its maker allows shortens its life sharply, which is a cost the runtime figure never shows.
Where this goes wrong
- Confusing in-lb and ft-lb. A factor of twelve. Applied to a small fastener it strips or snaps it; applied to a large one it leaves the joint dangerously loose.
- Converting a torque figure from another engine or another market. Look up the fastener, do not convert a number you already had.
- Entering full rated capacity as usable. Set the usable percentage to what the chemistry actually allows, or the runtime is roughly double the truth.
- Forgetting inverter losses when the load is an AC appliance.
- Solving Ohm's law for the wrong unknown. Pick the quantity you are missing from the dropdown; the two fields change to match.
- Working on a live system. Disconnect the battery before touching vehicle wiring. A 12 V system will not shock you and will very happily start a fire.
Where these numbers come from
Unit conversions and Ohm's law are exact. The value of getting them right is that a torque figure copied in the wrong unit is a failed fastener.
- NIST Special Publication 811 — The authoritative guide to SI units and conversion factors, including torque and the ones people habitually mix up.
- SAE International — Fastener and torque specification standards for vehicle service work.
Why not discharge a battery to 100%?
Deep discharge shortens the life of most lead-acid batteries significantly — many manufacturers recommend not routinely going below 50% state of charge, with 80% usable capacity as a common conservative planning number. Lithium (LiFePO4) batteries tolerate deeper discharge much better.
Does Ohm's Law apply to AC circuits too?
The basic V=IR relationship holds for resistive AC loads, but AC circuits with motors, inductors, or capacitors involve additional factors (impedance, power factor) that this simple calculator doesn't account for.
Estimates only. For torque specs, always use your vehicle or equipment manufacturer's exact figures — this tool converts units, it doesn't tell you what spec to use.