What is the general mass balance equation?

Accumulation = Input − Output + Generation − Consumption. At steady state, accumulation = 0, so Input + Generation = Output + Consumption.

What does accumulation mean?

Accumulation is the net rate at which mass builds up (or depletes) inside the system boundary. A positive value means mass is accumulating; negative means mass is leaving.

When is generation or consumption non-zero?

Generation and consumption are non-zero for reactive systems. For a non-reactive process (e.g., a mixer or splitter), both are set to zero and the equation simplifies to Input = Output + Accumulation.

What units should I use?

Any consistent mass or molar flow rate unit works — kg/h, g/s, lb/h, mol/s, etc. The calculator does not convert between units, so make sure all values are in the same unit before solving.

What if I get a negative result?

A negative accumulation means the system is losing mass (e.g., drawing down a tank). A negative input or output would indicate a data entry or sign-convention error — check that you have entered all values with the correct sign.

Mass Balance Calculator

Name: Mass Balance Calculator
Availability: InStock
Author: Nham Vu

Enter any three of the four mass balance terms and instantly solve for the unknown stream.

Mass Balance Inputs

Accumulation = Input − Output + Generation − Consumption

Solve for

Unit (display only)

Fill in the known values and press Calculate.

Accumulation

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Equation breakdown

All terms

Term	Value

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Summary

Enter any three of the four mass balance terms and instantly solve for the unknown stream.

How it works

Select which variable you want to solve for from the dropdown.
Enter values for all other known streams (input, output, generation, consumption, accumulation).
The tool applies the general mass balance equation: Accumulation = Input − Output + Generation − Consumption.
The unknown is calculated and displayed with the full equation substituted.
Use the unit selector to keep values consistent across kg, g, lb, or mol.

Use cases

Determine the accumulation rate inside a reactor given feed and exit streams.
Back-calculate an unknown input flow when outlet and accumulation are measured.
Verify a process design by checking that all streams balance to zero at steady state.
Solve textbook chemical engineering mass balance problems step by step.
Check material losses in a separation or distillation column.
Calculate generation or consumption terms for reactive processes.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu