Solution Preparation Calculator
From solid or from a concentrated stock — weigh the right amount, dilute the right volume.
Assay or hydrate content of the reagent. Leave at 100 for pure anhydrous material.
Extra percentage added to cover weighing and transfer losses. Use 0 for an exact amount.
n = C × V · m = n × M / (purity/100) × (1 + excess/100)
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What it does
Solution preparation is one of the most repeated calculations in a chemistry or biology lab, and one of the easiest to get wrong. The concentration you write on the bottle is a statement about moles per litre, but the practical question is how many grams to put on the balance or how many millilitres to pipette from a stock bottle. This calculator converts between the two views, keeping unit conversions explicit so that millimolar targets and microlitre volumes do not get mixed up. Purity and excess fields matter for real reagents: hydrates, salts assayed at 99%, and small weighing losses all shift the delivered concentration away from the nominal value.
When to use it
Use it when making up media salts, buffer stocks, antibiotic working solutions, or any reagent specified as a molar concentration. The solid mode is the common case for laboratory-grade powders, and the stock mode covers everyday dilutions from 1 M or 10× concentrates. It also serves as a quick sanity check before committing reagents: if the calculated mass looks implausible for the volume, the unit or the molar mass is probably wrong.
FAQ
- How do I calculate how much solid to weigh to make a solution?
- Multiply the molar concentration by the final volume in litres to get the number of moles, then multiply by the molar mass. For example, 100 mL of 10 mM NaCl (58.44 g/mol) needs 0.001 mol × 58.44 g/mol = 58.4 mg. Enter purity below 100% if your reagent is a hydrate or is assayed at less than 100%.
- What does the excess percentage mean?
- It adds a handling margin to the weighed mass so that pipetting losses and transfer residue do not leave you short of the target concentration. A 5% excess on 58.4 mg gives 61.3 mg. Use 0% when you need an exact stoichiometric amount.
- How much stock solution and diluent do I need?
- Use C1V1 = C2V2. Stock volume = (final concentration × final volume) / stock concentration, and diluent volume is the final volume minus the stock volume. For 100 mL of 10 mM from a 1 M stock, take 1 mL of stock and add 99 mL of diluent.
- Why can the calculator report an invalid result?
- A dilution is impossible when the stock is more dilute than the target, because the required stock volume would exceed the final volume. The tool flags this case instead of returning a negative diluent volume; check that the stock concentration is higher than the final concentration.