Grow box fan performance calculator
Ventilation defines the microclimate. Use this quick guide to calculate grow box volume, target fan performance, and choose filters.
Performance calculator
Enter dimensions and losses — results update automatically.
20 exchanges/hour is the 3-minute standard.
Typically 5–10% per 2–3 m; here it is per 3 m.
V = (X × Y × Z) / 1,000,000 m³
Results
Passive intake is often enough; active intake should be lower than exhaust.
1. Grow box volume calculation
- Convert sizes to meters: length = X/100 m, width = Y/100 m, height = Z/100 m.
- Volume V = (X/100) × (Y/100) × (Z/100).
- Quick formula: V = (X × Y × Z) / 1,000,000 m³.
V = (X × Y × Z) / 1,000,000 m³
2. Fan performance calculation (m³/hour)
Aim to fully exchange air every 3 minutes (20 exchanges/hour). Increase for hot/small boxes, reduce for extra odor filtration.
- Standard: every 3 minutes → 20 exchanges/hour.
- Hot or small box: every 1–2 minutes → 30–60 exchanges/hour.
- Maximum odor filtration: every 4–5 minutes → 12–15 exchanges/hour.
Base exhaust performance: V × 20 m³/hour.
Add loss coefficients:
- +20–30% for a carbon filter (resistance).
- +5–10% for each 2–3 m of ducting.
- +10% for each 90° bend.
- +10–20% if using cooltubes or extra lamps.
Example: with filter and 3 m of ducting → V × 20 × 1.3 × 1.1 ≈ V × 28.6.
Intake fan
Passive intake is usually enough. If active, size it at 60–80% of the exhaust to keep slight negative pressure.
Example (X=100, Y=100, Z=200 cm)
- Volume: V = (100×100×200) / 1,000,000 = 2 m³.
- Base exhaust: 2 × 20 = 40 m³/hour.
- With filter/ducting: 40 × 1.3 ≈ 52 m³/hour.
- Fan selection: 60–100 m³/hour with headroom.
- Intake: passive or active 30–40 m³/hour.
3. Fan recommendations
- Exhaust: inline duct fan (Vents TT, Blauberg, or similar) sized to the target flow.
- For quieter operation, add a speed controller (dimmer).
- Intake: smaller inline fan or passive intake.
- Keep 20–50% headroom for tuning and aging.
4. Filters
- Exhaust: carbon filter for odor control — essential for aromatic plants.
- Match filter performance to the fan (e.g., 100 m³/hour fan → 100–150 m³/hour filter).
- Filter diameter must match the fan (usually 100–150 mm).
- Intake: dust/pre-filter (mesh/HEPA); no carbon needed.
Tip: mount the filter before the exhaust fan inside the box and replace carbon every 6–12 months.
Quick calculation checklist
- Measure length, width, and height, then convert to meters.
- Calculate volume: V = (X × Y × Z) / 1,000,000.
- Get base exhaust: V × 20 exchanges/hour.
- Add losses for filter, ducting, bends, and heat.
- Choose an exhaust fan with headroom and 60–80% intake.
Expertise and trust
This guide is based on real grow box ventilation setups and manufacturer recommendations.
- We explain formulas so you can verify the math yourself.
- Loss coefficients reflect common filter and ducting installations.
- Recommendations are tailored to typical grow box sizes and fan models.
Sources and standards
- Technical specs for inline duct fans (Vents, Blauberg) and their performance curves.
- Manufacturer guidance on carbon filter pressure loss.
- Practical air exchange norms for compact ventilation systems.
Answers & questions
How much performance headroom do I need?
Plan for 20–50% headroom to offset losses, noise limits, and long-term wear.
When do I need active intake?
Use active intake for large boxes or long ducting. It should be weaker than exhaust.
How often should I replace carbon?
Typically every 6–12 months, or sooner if odor or airflow drops.
Author
GrowDiaries Team
Grow box ventilation and microclimate specialists
Over 7 years helping growers select ventilation, filters, and gear for home and commercial grow boxes.
Expert review
Ihor Shevchenko
Technical consultant for ventilation systems
Specializes in airflow design, inline systems, and filtration selection.