Emission Rate Calculator
Calculate source emission rate from measured indoor concentration, ventilation rate, and room volume.
About this calculator
This calculator works backwards from a measured indoor pollutant reading to figure out how strong the source actually is. It relies on the well-mixed room model's steady-state relationship, C_ss = E / Q, which says that once concentration stops changing, the emission rate must exactly equal concentration times ventilation rate: rearranged, E = C_net × Q. It first converts your ventilation input (CFM) and room volume (ft³) to metric units, subtracts your specified background/outdoor concentration from the measured reading to isolate the source's actual contribution, and multiplies that net concentration by the ventilation rate to get an emission rate in µg/hr (and mg/hr).
It also divides that emission rate by room volume to get "source strength" — a size-normalized figure useful for comparing sources across differently sized spaces — and reports the air exchange rate (room volumes of outdoor air per hour) plus the time needed to reach 90% of steady state, derived from the exponential buildup equation C(t) = (E/Q)(1 − e^(−Qt/V)) as t₉₀ = 2.303 × V/Q. The steady-state concentration is also recomputed from the derived emission rate as a self-consistency check — it should equal your net concentration input if the room really was at steady state when measured. The biggest assumption here is that "well-mixed" one: the model treats the whole room as a single uniform concentration, which breaks down near the source itself or in poorly circulated spaces, and the whole calculation is only valid if your measurement was actually taken at steady state rather than mid-buildup — a reading taken too soon after the source started will understate the true emission rate.
Inputs
Results
Emission Rate (μg/hr)
7,645.5
How to Use This Calculator
- Enter Measured Concentration (μg/m³), Ventilation Rate (CFM), and Room Volume (ft³).
- Set Background Concentration (μg/m³).
- Review the Emission Rate (μg/hr) result.
- Use Emission Rate (mg/hr) and Source Strength (μg/m³·hr) to inform your decision.
How the result changes with Measured Concentration (μg/m³)
| Measured Concentration (μg/m³) | Emission Rate (μg/hr) |
|---|---|
| 25 | 3,398 |
| 38 | 5,606.7 |
| 75 | 11,893 |
| 125 | 20,388 |
What each input means
- Measured Concentration (μg/m³)
- Measured indoor pollutant concentration at steady state in micrograms per cubic meter.
- Ventilation Rate (CFM)
- Total outdoor air ventilation rate in cubic feet per minute.
- Room Volume (ft³)
- Total room volume (length × width × height) in cubic feet.
- Background Concentration (μg/m³)
- Outdoor or supply air background concentration of the pollutant.
What each result means
- Emission Rate (μg/hr)
- Calculated pollutant emission rate from the indoor source.
- Emission Rate (mg/hr)
- Emission rate converted to milligrams per hour.
- Source Strength (μg/m³·hr)
- Emission rate normalized to room volume.
- Steady-State Conc. (μg/m³)
- Verification: steady-state concentration from computed emission rate.
- Time to 90% Steady State (min)
- Approximate time for the room to reach 90% of steady-state concentration.
- Air Exchange Rate (1/hr)
- Number of room volumes of outdoor air per hour.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersMeasured Concentration (μg/m³) = 50, Ventilation Rate (CFM) = 100, Room Volume (ft³) = 1500, Background Concentration (μg/m³) = 5 = 4 input(s) provided
- Calculate Emission RateEmission Rate = netConcentration * ventilationM3Hr7645.5 = 7645.5
- Calculate Emission RateEmission Rate = emissionRateUgHr / 10007.646 = 7.646
- Calculate Source StrengthSource Strength = emissionRateUgHr / roomVolumeM3179.98 = 179.98
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator subtract a background concentration before computing emission rate?
The steady-state relationship E = C_net × Q only holds for the concentration contributed by the indoor source itself, not the total measured reading, which also includes whatever pollutant is already present in incoming outdoor or supply air. Subtracting the background concentration you specify isolates the net rise attributable to the source, so an outdoor background that's ignored would cause the calculator to overstate how strong the actual indoor source is.
What does 'source strength' add beyond the raw emission rate in µg/hr?
Source strength divides the emission rate by room volume, giving a size-normalized figure in µg/(m³·hr). This lets you compare how strong two sources actually are independent of the room they happen to sit in — a source that looks weaker in raw µg/hr inside a small room could actually be a stronger emitter per unit volume than a source producing a higher raw number in a much larger space.
Why is there a 'steady-state concentration' output that just seems to recompute my input?
It's a self-consistency check, not a repeat of your input. The calculator derives an emission rate from your net concentration and ventilation rate, then independently recomputes what steady-state concentration that derived emission rate would produce (C_ss = E/Q). If your original measurement was genuinely taken at steady state, this recomputed value matches your net concentration input; a large mismatch would suggest the measurement wasn't actually at steady state when taken.
How do I know if my measurement was actually taken at steady state, and why does it matter?
The calculator reports the time to reach 90% of steady state (t₉₀ = 2.303 × V/Q) so you can compare it against how long the source had been running before you took your reading. If you measured concentration before that time had elapsed, the room was still mid-buildup rather than at steady state, and the whole E = C_net × Q calculation — which assumes steady state — will understate the true emission rate.
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