Dial in humidity goals with clear inputs for rooms, climate, and airflow. Get required output, unit count, and margin recommendations for reliable performance year-round.
| Scenario | Room | Air exchange | Indoor target | Outdoor | Safety | Estimated output |
|---|---|---|---|---|---|---|
| Office fit-out | 120 m² × 2.8 m | 1.0 ACH | 22°C, 40% RH | 5°C, 60% RH | 15% | ≈ 40–70 L/day (varies by pressure and airflow) |
| Dry warehouse | 500 m² × 6.0 m | 0.6 ACH | 18°C, 35% RH | 0°C, 70% RH | 20% | ≈ 150–300 L/day (depends on leakage) |
| Workshop | 250 m² × 3.5 m | 700 m³/h | 20°C, 45% RH | 10°C, 50% RH | 10% | ≈ 90–160 L/day (depends on internal moisture) |
This calculator converts temperature and relative humidity into humidity ratio w (kg water per kg dry air).
Air exchange is entered as ACH or a flow rate, then converted to mass flow using density at indoor conditions.
Heated outdoor air keeps the same moisture content, so the outdoor humidity ratio is used as the incoming air moisture level.
Assumptions: steady-state mixing, no dehumidification, and no moisture buffering from materials. For large projects, verify with a full HVAC design workflow.
Indoor relative humidity (RH) affects worker comfort, material stability, and finish quality. Many fit-outs aim for 30–50% RH during occupied operation. During curing and commissioning, short-term targets may differ, but undersizing can prolong dry-air conditions that drive cracking, shrinkage, and static discharge.
Humidifier demand is highest when outdoor air is cold and dry in moisture content. Even if outdoor RH looks high, cold air can hold little water vapor. For planning, evaluate a winter design point (for example, 0–10°C) and your expected outdoor RH, then let the calculator convert that to humidity ratio.
Construction phases often have elevated leakage. If you do not have measured ventilation rates, start with 0.5–2.0 ACH for typical conditioned spaces and test higher values for very leaky shells or frequent door cycling. Because load scales with airflow, doubling ACH roughly doubles required humidifier output.
The calculator uses room volume (area × height, or direct volume) and converts ACH to a flow rate. If you have a mechanical schedule, entering m³/h (or CFM) is often more accurate than ACH. For large spaces, break the project into zones and size each zone to avoid uneven humidity control.
People and processes add moisture. A light-activity occupant can contribute roughly 40–100 g/h depending on metabolic rate and conditions. Wet trades, wash-down, or temporary curing can add more. Enter realistic gains to avoid overestimating humidifier capacity.
Manufacturers commonly publish output in kg/h, lb/h, L/day, or gal/day. This calculator reports all three common units so you can compare models quickly, then estimate unit count from a chosen single-unit capacity. Always confirm published output at your steam, water, and air conditions.
A practical buffer is 10–25% to cover infiltration swings, door events, and control deadband. High-precision environments may require tighter control with staged outputs. If the project has uncertain envelope tightness, select a higher safety factor and re-check after blower-door or commissioning measurements.
Verify RH sensors, confirm airflow assumptions, and inspect for condensation risk at cold surfaces. Maintain clear setpoints and ensure drainage, water quality, and service access are planned. A correctly sized humidifier reduces callbacks and supports consistent interior conditions through handover.
Most commercial and residential spaces target 30–50% RH for comfort and material performance. Cold climates may run lower to reduce condensation risk on windows and thermal bridges.
Cold air holds less water vapor. When that air is warmed indoors, its relative humidity drops sharply unless moisture is added. Humidity ratio captures the true moisture content.
Use flow rate when you have mechanical ventilation data. Use ACH for early-stage estimates or when infiltration dominates. If unsure, run multiple ACH scenarios to bracket demand.
People add moisture through breathing and perspiration. Light activity commonly contributes 40–100 g/h per person. In small rooms, this can noticeably offset required humidifier output.
A 10–25% margin is common for construction variability, door events, and leakage uncertainty. Use higher margins if envelope tightness is unknown, then refine after commissioning measurements.
If the outdoor humidity ratio is already higher than your indoor target, or internal moisture gains exceed ventilation drying, additional humidification may not be required under that scenario.
No. It is a fast sizing and comparison tool. Final selection should consider distribution method, control strategy, condensation risk, water treatment, and manufacturer performance data.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.