Room Air Exchange and Ideal Mixing Calculator
Convert entered replacement airflow to air changes per hour and model the fraction of original room air remaining under perfect mixing.
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Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Divide effective replacement flow by room volume. Under constant flow, perfect mixing and no new source of the modeled original-air tracer, its fraction decays exponentially. Reverse the decay equation for an entered remaining fraction.
ACH=effective m³/hour/room m³; remaining fraction=exp(−ACH×hours); time=−ln(target fraction)/ACH
Worked example
A 50 m³ room with 100 m³/h replacement flow and an entered effective fraction of 80% gives 1.6 ACH. After 30 minutes, about 44.933% of the original-air tracer remains; reaching an entered 10% takes about 86.347 minutes.
Assumptions and input details
Ideal single-zone mixing model, not an air-quality measurement, health threshold or safe re-entry time. Use replacement flow that removes the modeled tracer, not ordinary recirculation without removal. Effective fraction is entered and not measured here. Constant flow, zero incoming tracer and no continuing source are assumed; dead zones, outdoor pollutants and changing conditions are excluded. One air change does not mean every original molecule is replaced. Perfect zero remaining is not reached in finite time. Zero effective flow leaves the original fraction unchanged.
References: NIST: tracer-gas decay and building air exchange
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
The 36 volume definitions distinguish US liquid, US dry and Imperial measures. Cups carry an explicit definition. Japanese shō is 2401/1331 L and a gō is one tenth of it; the explicitly labelled 180 mL rice cup is separate.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · UK Imperial definitions · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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