Inputs
Results
Required injector size (cc/min) = HP × BSFC × 10.5 × overhead ÷ (n × duty × 0.6). lb/hr → cc/min: × 10.5.
What this calculator actually solves
Injector sizing is a fuel-flow problem. Horsepower × BSFC gives you the total fuel the engine burns per hour at that horsepower. Divide by injector count and you get per-injector flow. Add a margin for transient enrichment, idle quality, and the fact that injectors lose flow over time as deposits accumulate, and you have a defensible injector size.
Most calculators stop at the lb/hr number. This one also reports cc/min (the unit most aftermarket injectors are sold in) and flags a duty-cycle issue if the calculated size would force the injectors to operate above your target duty cycle.
Who this is for
Engine builders and tuners choosing injectors for a stand-alone ECU. Especially useful when picking between two sizes (say 60 lb/hr vs 80 lb/hr) and wondering which actually fits the duty-cycle budget.
Turbo and nitrous applications where transient enrichment matters more than steady-state.
How to read the results
Pick the next standard injector size above the calculated requirement. If you land between two standard sizes, the larger one gives you transient margin; the smaller one will leave you with high duty cycles at peak.
A duty-cycle flag of "below 80%" is safe. Above 85%, the injectors are getting close to their mechanical limit and idle quality can suffer.
Limitations
BSFC varies with load. At wide-open throttle it's lower (~0.45 lb/hp·hr for a naturally aspirated gas engine); at idle it can exceed 0.65. We use your BSFC value as a single number, which is correct for peak WOT but may understate the per-cylinder transient requirement.
E85 and methanol need ~30–50% more fuel than gasoline. If you're running alcohol, multiply the result by 1.3 to 1.5.
Frequently Asked Questions
What BSFC should I use?
For a stock to mild naturally aspirated gas engine at WOT, 0.46–0.50. For a forced-induction gas engine, 0.50–0.55. Race-only engines with aggressive timing sometimes drop to 0.42 — but they also run hotter, so 0.45 is a safer mid-point.
Why is duty cycle limited to 85%?
Above ~85% the injector solenoid's linear flow region ends and the injector starts behaving non-linearly — small pulse-width changes produce large flow changes. Below 85% the flow-vs-pulse-width curve is predictable, which is what your ECU needs.
Is dead-time compensated here?
No. Dead-time is the brief delay between the ECU commanding the injector open and fuel actually starting to flow. It varies with fuel pressure. Your ECU's tune has to compensate for it — that's a calibration issue, not a sizing issue.
About this tool
What this tool is for: Fuel Injector Sizing Calculator is a workbench reference for working mechanics, machinists, restoration shops, and serious hobbyists. It is built because the usual online calculators skip the friction loss, the sign convention, the unit conversion, or the engineering code that actually matters on the job.
Purpose and scope
Required fuel-injector lb/hr for a target horsepower with a BSFC assumption and a duty-cycle headroom. It does not consider fuel-pressure corrections, pulse-width slew rate, or atomization quality at low pulse-widths.
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The sections above this footer — “What this calculator solves,” “Who this is for,” “How to read the results,” and “Limitations” — describe in detail what the formula does, why a tech would use it, how to interpret the number, and where the model is wrong. The FAQ block answers the three or four questions most asked about this specific tool. Together they make up the “About” content for this page; you do not need to look elsewhere.
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