Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.

Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.

Automotive Calculators

Parasitic Draw Calculator

2026-09-27 6 min

Days-to-drain and daily percent loss from key-off draw in milliamps and battery size.

Days to 50% Charge: 29.2 days; Below this, cranking suffers
Drain Per Day: 1.7 %/day

Calculator

Key-Off Draw(mA) Help

Measured after sleep (20+ min)

Battery Size(Ah)

Results update live as you type

Live statistics

Days to 50% Charge

29.2 days

Below this, cranking suffers

Drain Per Day

1.7 %/day

Results

Optimal

Days to 50% Charge

29.2days

Below this, cranking suffers

Drain Per Day

1.7%/day

Planning estimate from standard automotive formulas. Validate with dyno, timeslip or datalog data when precision matters.

Visualizations

Results overview

29.2Days to 50% Charge

1.7Drain Per Day

Days to 50% Charge vs. Key-Off Draw

Days to 50% Charge

29.2days

060

Formula

Converts measured dark current into calendar survival time against the 50%-charge crankability floor.

Days to 50% = (Ah x 0.5) / amps / 24

Alternative Form

Normal draw: 20-50 mA after modules sleep (~20-40 min)

Variables

Symbol	Name	Unit	Description
mA	Dark current	mA	Post-sleep key-off draw

Explanation

Modules stay awake ~20-40 min after lock; measuring too soon fakes a draw. True faults (stuck relays, modules, trunks lights) read hundreds of mA.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Record the inputs.

Key-Off Draw:50 mA

Battery Size:70 Ah

Result: 2 inputs ready

Step 2

Apply the formula.

Days to 50% = (Ah x 0.5) / amps / 24

Result: Primary result = 29.2 days

Step 3

Read every derived output.

Days to 50% Charge:29.2 days

Drain Per Day:1.7 %/day

Result: Days to 50% Charge = 29.2 days

Final Answer

29.2

Worked Examples

See how the calculator works with different scenarios.

Scenario

Normal modern car parked at the airport.

Inputs

Draw:50 mA

Battery:70 Ah

Calculation

Days = 35/0.05/24 = 29 days; 1.7%/day

Result

~29 days to half charge.

Interpretation

Fine for trips; a tender for months-long storage.

Understanding Your Result

Your Result: Days to 50% Charge: 29.2 days

Typical range: Under 50 mA: normal. 50-100: watch. Over 100: find the drain.

What This Means

How long the car can sit before the battery gives up.

Why It Matters

Diagnoses overnight-dead batteries: math or measurement, one of them is wrong.

Professional Interpretation

Treat the primary result as a planning estimate, then validate on a dyno, at the track, or with logged sensor data. If measured numbers disagree by more than ~10%, re-check inputs (units are the usual culprit) before changing parts.

Recommendations

•Double-check units on every input before trusting the result
•Compare against the worked example below to sanity-check magnitude
•Validate with measured data (dyno, timeslip, datalog) when it matters

Learn — fuel economy, deeply

Parked cars sip current for clocks, alarms and module memory.

1 · Where the gallon goes (energy path)

Engine heat

60–70% lost

Coolant + exhaust carry most energy away. Hybrids recover some via Atkinson cycle + regen.

Driveline

5–15% lost

Transmission, differential, accessories. CVTs and 8–10 speed autos lose less.

Aero drag

∝ v²

Doubles from 55→78 mph. Roof boxes hurt more than trunks.

Rolling + brakes

10–20%

Tires, weight, stops. Regen braking (hybrid/EV) reclaims city braking energy.

4 · Measure like a pro (the 3-tank protocol)

Fill to first click at the same pump, record the odometer (Tank 0).
Drive normally for 2–3 tanks; always refill to first click at the same pump.
MPG = total miles ÷ total gallons across all tanks — averaging kills pump-click noise.
Cross-check with the trip computer; a persistent >10% gap means the computer needs calibration — or the car needs service.
Log with an OBD-II dongle for per-trip MPG if you want to isolate commute vs. highway behavior.

Practical Tips

•Wait 30+ min for sleep before measuring
•Start with trunk/glovebox lights
•Use a tender past 3 weeks parked

Common Mistakes

Mixing units (inches with mm, psi with kPa)

Convert every input to the unit shown on its field before calculating.

Trusting a single output blindly

Cross-check with the worked example and, when it matters, a dyno or datalog.

Assumptions

This calculator makes the following assumptions to provide estimates:

Steady-state model: Formulas assume stabilized conditions; transients, heat soak and traction limits are excluded.

Standard factors: Empirical constants (BSFC, VE, drivetrain loss) use typical values — your combination may differ.

Limitations

Be aware of these limitations when using this calculator:

Estimate, not measurement: Predictions carry real-world scatter of roughly ±5–15% depending on the model.

No transient effects: Shift time, wheelspin, spool lag and heat soak are not modeled here.

Frequently Asked Questions

Answers to common questions about this calculator.

What is normal parasitic draw?

20-50 mA after full sleep. Over ~85-100 mA will kill a battery in days to a couple of weeks.

How do I find a battery drain?

Ammeter in series after sleep, then pull fuses one by one — the circuit whose removal drops the reading is the culprit.

Related Calculators

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Related Guides

Deepen your understanding with these educational resources.

Automotive Guides

Build planning, measurement protocol and validation how-tos

Glossary

Technical terms used in mortgage calculations explained in plain language.

MPG: Miles per gallon. Distance travelled per gallon of fuel; higher is better. US gallon = 3.785 L.
L/100 km: Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.
MPGe: Miles per gallon-equivalent. EV metric: miles travelled on 33.7 kWh (energy of 1 gal of gasoline).
CAFE: Corporate Average Fuel Economy — US fleet-average efficiency standard enforced by NHTSA.
Rolling resistance: Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.
Aerodynamic drag: Air resistance growing with the square of speed — why highway MPG falls fast above 60 mph.
Hypermiling: Driving techniques (coasting, steady speed, anticipation) that maximise real-world MPG.
OBD-II: On-board diagnostics port. Scanners and dongles read live MPG, fuel trim and fault codes.
Dark current: Key-off electrical draw after all modules enter sleep mode.

Authoritative References

Trusted sources and standards referenced by this calculator.

EPA — FuelEconomy.gov DOE — Fuel-Efficient Driving Tips NHTSA — Driving Safety & Efficiency SAE International — Fuel Economy Standards Consumer Reports — MPG Truth IRS — Standard Mileage Rates

Applicable Standards

EPA Fuel Economy Testing (40 CFR Part 600)CAFE — Corporate Average Fuel Economy Standards (NHTSA)WLTP — Worldwide Harmonised Light Vehicle Test ProcedureSAE J1321 / J1711 — Fuel consumption test proceduresISO 2575 — Road vehicles symbols & efficiency markings

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Page Information

Last Updated:

2026-09-27

Version:

2.0.0

References:

License:

Free to use for educational and professional purposes.