How full should I fill my portable generator’s fuel tank?

Environment

Applicable to Generac portable generators. Always consult your specific model’s owner’s manual for exact instructions.

Short answer

Fuel fill heights can vary depending on your generator model. Please refer to your unit's owner's manual for exact recommendations on fuel fill heights.

IMPORTANT: Overfilling can force fuel into the venting or evaporative system or spill onto hot engine parts, which can lead to a fire or explosion. Wipe up spills immediately and tighten the fuel cap before restarting.

Why headspace matters

• Thermal expansion: Gasoline expands as it warms. Leaving headspace prevents pressurization, leaks, and odors.
• Evaporative system protection: On models equipped with carbon canisters or rollover valves, overfilling can saturate components and cause hard starting or rough running.
• Level accuracy: A level, un-overfilled tank helps the fuel gauge (if equipped) read more consistently.

Safe refueling steps

1. Shut down and cool: Turn the generator OFF. Let it cool for at least 2–5 minutes before refueling.
2. Move outside: If the generator is in storage, move outdoors to a well-ventilated area before refueling.
3. Level surface: Place the generator on a level surface to avoid uneven filling and splash back.
4. Use proper fuel: Only use clean, fresh unleaded gasoline with a minimum octane rating of 87 and no more than 10% ethanol. Do not use E85, do not use an oil and gas mix. 
5. Fill slowly: Insert the nozzle slowly, fill to the “MAX/FULL” line or leave about 1/2 inch of headspace, and stop if fuel backs up in the neck.
6. Secure and clean: Tighten the cap securely. Wipe up any spills. Move the unit to its operating location before restarting.
7. Store fuel safely: Keep fuel in approved containers, away from living areas and heat sources.

Find your exact fuel capacity and instructions

For model-specific fuel type, capacity, and refueling procedures, refer to your owner’s manual. If you need help locating the correct manual, visit the product lookup page.

Related Help Center guidance

• Fuel type and ethanol guidance for portable generators
• Storing your portable generator and gasoline
• Carbon monoxide (CO) safety for portable generators

Need service or have questions?

If you experience fuel leaks, persistent fuel odors, or starting issues after refueling, discontinue use and contact an authorized Generac dealer. Use our dealer locator to find support near you.

See also

• Safe refueling steps (above)
• Model-specific notes (above)
• Find your Owner’s Manual
• Authorized dealer support