Grow Light Run Cost Calculator
Monthly and yearly electricity cost to run a grow light, by watts, hours and rate.
Use ToolFree calculator
Grow Room Fan CFM Calculator
Exhaust fan size in CFM to exchange a grow tent or room's air on schedule.
Grow Room Fan CFM Calculator
Chlorova · updated for 2026
Calculator
Quick samples
Click “Calculate” to see your results.
CFM here is free-air flow; every foot of ducting, each bend and any carbon filter drops a fan's real output, often by twenty to forty percent, so always size against the fan's rated curve with your filter attached rather than the box number. Passive intake must be able to feed the exhaust, or the tent holds negative pressure that stalls the fan and it moves a fraction of the figure shown here.
The grow room fan CFM calculator sizes the exhaust you need to turn over a tent or room's air on schedule, which is the number that decides whether your grow stays cool or cooks. Enter the room's length, width and height and pick a venting need, and it returns the cubic footage, the minimum exhaust CFM, and a higher figure to allow for a carbon filter. Undersized ventilation is the most common silent failure in indoor growing — it shows up as heat stress and mold long before anyone thinks to check the fan rating.
How to Size an Exhaust Fan
- Choose how hard the room is vented — standard, hot, or filtered.
- Enter the grow space's length in feet along its longest wall.
- Enter its width in feet across the canopy footprint.
- Enter the grow height in feet, measuring to the canopy, not the ceiling.
- Read the room volume and the base exhaust CFM it needs.
- If you fit a carbon filter, always use the higher filter CFM figure.
- Round up to the next commercial fan size you can actually buy.
- Match the passive intake so the exhaust is never starved of make-up air.
How Exhaust CFM Is Worked Out
Volume is length times width times height in cubic feet. A grow wants the whole air changed about once a minute, so the base exhaust is the volume times the air changes per minute you selected — one for a standard tent, one-and-a-half for a hot room, and two where a carbon filter is in play. A filter strangles airflow, so the recommended figure adds twenty percent to make sure enough still actually moves through the canister.
Why a rating on the box is not the real number: fans are sold on free-air CFM measured with nothing attached, and duct length, elbows and especially a carbon filter each claw back a chunk of that flow. That is why the filtered result runs higher than the bare need — you are buying a fan whose degraded output with everything hooked up still clears the true target. Sizing to the raw number and then adding a filter is the classic way to end up with a tent that never reaches its rated ventilation.
Worked Example: A 4 × 4 Tent With a Filter
- Your grow tent measures 4 feet by 4 by 6 tall.
- Volume: four times four times six equals ninety-six cubic feet.
- A standard tent at one change a minute needs about 96 CFM.
- You run a carbon filter, so pick the filtered setting at two changes.
- Base need doubles to about 192 CFM before the filter penalty.
- Add the twenty percent allowance: roughly 230 CFM through the canister.
- Round up to the nearest common four-inch inline fan at or above that.
- You leave an intake vent at least as large as the exhaust so the fan is never starved of air.
- Duct the run short and straight — every extra elbow takes flow back off the fan's rating.
Common Tent Sizes and Exhaust
Base and filtered exhaust needs for common tent footprints at standard height, to sanity-check your own figures. Taller rooms and hotter lights push the number up, and any carbon filter means you should size toward the higher column rather than the bare CFM.
| Tent | Volume cu ft | Filtered CFM |
|---|---|---|
| 2 × 2 × 4 | 16 | 38 |
| 2 × 4 × 5 | 40 | 96 |
| 4 × 4 × 6 | 96 | 230 |
| 4 × 8 × 7 | 224 | 538 |
Key Terms
- CFM
- Cubic feet per minute — how much air a fan moves, and the number you buy an exhaust on.
- Air change
- One full replacement of the room's air volume; growers target roughly one every minute.
- Carbon filter
- A charcoal canister on the exhaust that scrubs odor but sharply resists and reduces airflow.
- Static pressure
- The resistance a fan pushes against from duct and filter, which cuts its real delivered CFM.
- Passive intake
- The make-up air opening that feeds the exhaust; undersizing it stalls the whole system.
What This Means When You Buy a Fan
- Buy against the fan's rated curve with your filter and duct fitted, not the free-air number on the box — that headline CFM is measured with nothing in the way.
- A carbon filter is the single biggest airflow thief in a tent, often dropping a fan thirty to forty percent; the two-change setting exists so the exhaust still clears the target once the canister is on.
- Exhaust moves more than heat — it resets CO₂ and humidity, so a stuffy tent that never fully turns over invites botrytis even at comfortable temperature.
- Keep the grow height honest: measure the air the plants and lights actually occupy, not the full room ceiling, unless the ceiling is genuinely part of the plenum.
- Negative pressure is the goal — the tent should suck in a little at the intake and hold a slight vacuum so unscrubbed air never leaks out, which means a slightly larger intake than exhaust.
Common Mistakes & Fixes
- Sizing to the bare once-a-minute CFM and then bolting on a carbon filter that quietly halves the real airflow.
- Trusting the free-air box rating instead of the manufacturer's curve at the actual static pressure of the run.
- Making the passive intake too small, so the exhaust cannot pull enough air and the tent stalls under negative pressure.
- Measuring to the ceiling instead of the canopy on a tall room, wildly oversizing the fan and wasting money and heat.
- Running the fan at full speed around the clock and drying the canopy edges, when a controller on a humidity or temperature target fits the need far better.
When to Use This (and When Not To)
Use it whenever you set up or reconfigure a tent or enclosed grow room and need the exhaust figure before shopping. It sizes air exchange only; heat from your lights drives the real requirement, so if a big hot fixture is cooking the room, cut the heat load at the source rather than buying an enormous fan. For whole-house or garage grows with complex ducting and multiple rooms, work the layout out with a ventilation guide, since long duct runs change the answer more than the volume does.
Grow Room Fan CFM Calculator: Frequently Asked Questions
How many CFM do I need for a grow tent?
Roughly the tent's volume in cubic feet exchanged once a minute, so a 4 × 4 × 6 tent wants near 96 CFM. Add about twenty percent more for a carbon filter, and size up further on a hot room or heavy light load.
Does a carbon filter change the fan size?
Yes, and a lot. A carbon canister resists airflow and can drop a fan's delivered CFM by a third or more, so you buy a bigger fan than the bare room needs. The calculator's filtered column builds that allowance in for you.
Should the fan run all the time?
The exhaust usually runs continuously to keep air turning over, but speed is best managed on a controller against a temperature or humidity target. A fan stuck at full blast around the clock dries the canopy edges and wastes electricity.
What size intake goes with my exhaust?
Make the passive intake at least as big as the exhaust opening, and often larger, so the fan is never starved. If the tent cannot pull enough make-up air, it runs under too much negative pressure and delivers only a fraction of its rated CFM.
Is one air change a minute always enough?
It is a good starting target for a typical tent, but hot rooms, high-heat lights and strong odor control all push you toward one-and-a-half or two changes a minute. The drop-down here covers those cases; when in doubt, round up.
Why is my box CFM higher than my real airflow?
Because the box states free-air CFM with no duct, bends or filter attached. Every bit of added resistance eats flow, so the same fan might move only sixty to eighty percent of that number in a real install. Size from the fan's pressure curve instead.
Sources & Further Reading
- Ventilation and Air Exchange for Indoor Grow Rooms — University of Florida IFAS Extension
- Sizing Exhaust Fans and Carbon Filters in Controlled Environments — Oregon State University Extension
- Greenhouse and Grow Room Airflow Fundamentals — Cornell University Controlled Environment Agriculture
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated
More popular tools
All toolsNew New Grow Tent Circuit Breaker Calculator
How many grow lights, fans and pumps safely fit on a 15 A or 20 A circuit.
Use ToolNew Mushroom Substrate Calculator
Wet substrate weight, flush yield, pasteurization hours by method and a colonization finish date for a dry-weight recipe.
Use ToolNew Grow Tent Heat Load Calculator
Turn fixture wattage into BTU per hour, then size the exhaust that holds your temperature rise and check the fan you already own.
Use Tool