What this calculator does
Conservation of mass is one of the oldest results in physics and chemistry: in a closed system, mass is neither created nor destroyed, only rearranged. The total mass going into a process (reactants in a chemical reaction, parts before an assembly, material before a physical change) equals the total mass coming out, even when that mass changes form completely, such as a solid reacting to release a gas.
This calculator does not compute anything exotic; it is one equation, mass before equals mass after, used to find whichever single mass is missing from an otherwise balanced system. That is genuinely how the law gets used in practice: a chemistry student knows the mass of every reactant and every product except one, and conservation of mass fills in the gap.
The formula
Add up every known mass on the "before" side, and every known mass on the "after" side excluding the one unknown quantity. Because total mass before must equal total mass after, the missing mass is simply the difference between those two totals.
| Term | Meaning |
|---|---|
| Mass before | The sum of all known masses entering the system, such as the reactants in a reaction. |
| Known mass after | The sum of all known masses leaving the system, excluding whichever one is unknown. |
| Missing mass | The one unknown mass that makes the two sides balance. |
The inputs explained
| Field | What to enter |
|---|---|
| Total mass before (sum of known reactant/part masses) (g) | The total of every known mass going into the system. |
| Known mass after (sum of known products/parts, excluding the unknown) (g) | The total of every known mass coming out of the system, not counting the missing amount you are solving for. |
When to use it
Balancing a chemical reaction by mass
Given the masses of all reactants and all products except one, conservation of mass finds the missing product or reactant mass directly, without needing the balanced chemical equation itself.
Checking a mass balance in an industrial process
Material entering a process (raw ingredients, feedstock) should equal material leaving it (product, waste, emissions); a mismatch signals measurement error or an unaccounted-for loss.
A physics or chemistry homework check
Working backwards from total mass before and after is a quick way to check that a more detailed calculation has not lost or gained mass somewhere along the way.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What is the missing mass at different starting totals?
A fixed 32 g of known product mass, against a range of total starting masses.
Questions
Does conservation of mass apply to nuclear reactions too?
Not exactly. In nuclear reactions a small amount of mass converts to energy (E = mc²), so mass alone is not conserved, only mass and energy together. For ordinary chemical reactions and physical processes, the small energy changes involved make mass conservation an excellent approximation.
What does it mean if the missing mass comes out negative?
It means the known mass after the process is already larger than the total mass before it, which is not possible in a closed system. That points to a measurement error, an input mistake, or mass entering from outside the system that was not accounted for.
Is this the same as balancing a chemical equation?
Related but not identical. Balancing an equation tracks the number of atoms of each element; conservation of mass tracks total mass regardless of what elements or compounds are involved. Both must hold simultaneously in a correctly balanced reaction.
Does this work for an open system, like something losing gas to the air?
Only if every mass leaving the system, including any gas released, is accounted for in the totals. If gas escapes unmeasured, the system will appear to have lost mass that conservation of mass cannot actually explain away.
For the energy side of a reaction or process rather than the mass side, see the kinetic and potential energy calculator.