Mass percent is the simplest way to state a concentration: how many grams of solute sit in every 100 grams of solution. Because it only needs a balance — no molar masses, no volumes — it is the figure on most reagent bottles, food labels and brines. The one thing to get right is the denominator: mass percent uses the total solution mass, which is the solute plus the solvent, not the solvent alone. Get that straight and the rest is a single division times 100.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the definition of mass (weight) percent.
The mass-percent equations
Keep both masses in the same unit; since it is a ratio, the units cancel and leave a percentage. The denominator is always the whole solution — solute plus solvent — which is why 20 g of solute in 80 g of water gives 20% (out of 100 g total), not 25%. To prepare a target strength, multiply the percent by the total mass you want to make: that gives the solute to weigh, and the remainder is solvent.
Worked example — making a brine
Scenario: You dissolve 20 g of salt in 80 g of water. What is the mass percent, and how would you make 250 g of the same brine?
The brine is 20% salt by mass. To make 250 g of it you weigh 50 g of salt and 200 g of water, keeping the same 1:4 solute-to-solvent ratio. If you instead wanted a 10% brine from the original 20 g of salt, you would need a 200 g solution — so you'd add water until the total reached 200 g, i.e. 180 g of water.
Frequently Asked Questions
(solute mass ÷ solution mass) × 100. 25 g in 100 g solution = 25%. Solution = solute + solvent.
Solution mass = solute + solvent (total). Mass percent is the solute's share of that total, out of 100.
Mass percent is g solute per 100 g solution (no molar mass). Molarity is mol/L (needs molar mass and volume).
solute = (%/100) × solution mass. 10% of 250 g = 25 g solute + 225 g solvent.
w/w = mass/mass (this tool); w/v = g per 100 mL; v/v = mL per 100 mL. Check which a label means.