Percent is the oldest way to describe a solution, and also the easiest to get wrong, because one word hides three different recipes. Five percent weight per volume means 5 grams in every 100 millilitres. Five percent volume per volume means 5 millilitres in every 100 millilitres. Five percent weight per weight means 5 grams in every 100 grams. The number is the same, but the amount you measure is different, and mixing the recipes up is a classic lab error.
This calculator keeps the three recipes apart. Pick the percent type first, then enter the percent and the total amount. For weight per volume and volume per volume, the total is in millilitres. For weight per weight, the total is in grams. The tool returns the solute to measure and the solvent that makes up the rest. If your protocol says 5 percent w/v and you need 200 millilitres, the answer is 10 grams of solute, because 5 percent of 200 is 10.
Weight per volume is the workhorse of school labs, because solids are weighed in grams and liquids are measured in millilitres. A 10 percent w/v salt solution needs 10 grams of salt in a final volume of 100 millilitres. Note the words final volume: you dissolve the salt and then fill up to 100 millilitres, you do not add 10 grams to 100 millilitres of water, or the total passes 100 and the percent drops. The same rule holds for every percent recipe.
Volume per volume is for two liquids. A 10 percent v/v alcohol solution needs 10 millilitres of alcohol in a final volume of 100 millilitres. Here the maths is simple, but the measuring needs care: small volumes need a pipette or a graduated cylinder, not a beaker, because beakers are for rough measuring. Temperature matters a little too, since liquids expand when warm, so measure at the temperature your protocol assumes.
Weight per weight is for work where volume would mislead, often with thick liquids or solids mixed into solids. A 10 percent w/w mixture needs 10 grams of solute in a total of 100 grams, so 10 grams of solute plus 90 grams of the other part. The total is a mass, not a volume, so a balance does the whole job and no cylinder is needed. That is why w/w shows up in ointments, alloys, and food recipes as well as in labs.
A worked check keeps you honest. Suppose a protocol asks for 250 millilitres of 8 percent w/v solution. Eight percent of 250 is 20, so weigh 20 grams and fill to 250 millilitres. Now check backwards: 20 grams in 250 millilitres is 20 divided by 250, which is 0.08, or 8 percent. That round trip, forward to the amount and back to the percent, catches typing slips in seconds, and it is worth doing on paper for every solution you plan.
Use this calculator as a practice aid for study and homework, not as lab procedure. It is an educational estimator, and real solution work follows a written protocol, a supervisor, and the safety data sheet for each chemical. Percent labels in a protocol must match the recipe you use exactly, and when a label is missing or unclear, ask before you measure. Check every result against your protocol, and never prepare a real solution from a web tool alone.
Diluting a percent solution follows the same conservation idea as dilution in general: the solute amount stays put while solvent is added. To turn 100 millilitres of 20 percent w/v solution into 5 percent, first find the solute: 20 percent of 100 is 20 grams. That 20 grams must become 5 percent of the final volume, so the final volume is 20 divided by 0.05, which is 400 millilitres. Add solvent up to 400 millilitres, not 300 millilitres on top of the original, because the target is a final volume of 400. The same division works for v/v and w/w, with millilitres or grams as the total, and it is worth checking by the round trip: 20 grams in 400 millilitres is 5 percent.