Stock Solution Calculator
A stock solution is a concentrated solution you prepare once and dilute later. Tell this calculator the concentration you want — as a molarity or as a percent weight/volume — the final volume, the solute’s molecular weight, and its purity, and it returns the exact mass of solid to weigh out, already corrected for purity, plus the moles involved and the steps to make it up.
Making a stock is where the molarity calculator and a real bench workflow meet. You rarely want 1 L of working solution from scratch; you want a small, concentrated stock — 1 M Tris, 10% SDS, 5 M NaCl — that you aliquot, store, and dilute on demand. This tool focuses on that job and adds the two things a bare molarity calculation leaves out: support for **percent weight/volume** stocks and an explicit **purity correction** from the reagent label. It sits in our laboratory calculator set and feeds straight into the dilution calculator and the C1V1 calculator for the working solutions you make from the stock.
If you need the molecular weight of the solute first, derive it from the formula with the molar mass calculator rather than guessing whether the bottle holds the hydrated or anhydrous form — that single choice changes the mass you weigh by tens of grams per mole. For the concept behind the concentration itself, what is molarity explains moles per litre and why final volume, not added water, is what counts.
Stock Solution Calculator
How to use the stock solution calculator
Choose the **stock type**. "Molar" prepares a stock defined in mol/L and needs the molecular weight; "Percent (% w/v)" prepares a stock defined as grams of solute per 100 mL of final solution and needs no molecular weight.
Enter the target concentration (molarity, or % w/v) and the **final volume** of stock you want, picking the volume unit. The calculator converts to litres internally, so you can think in mL at the bench.
Enter the solute **purity** from the label. If your solid is 95% pure, the mass is scaled up automatically so the active content is correct — leaving it at 100 assumes a fully pure reagent.
Press **Calculate**. The primary card gives the mass to weigh in grams (and milligrams); the secondary cards show the moles for a molar stock, the purity correction applied, and the final volume. Then dissolve in roughly 80% of the target volume and bring it up to the mark.
Stock Solution formula
Variables and units
| Symbol | Meaning | Unit |
|---|---|---|
| m | Mass of solute to weigh | g |
| M | Target molarity | mol/L |
| V | Final volume of stock | L |
| MW | Molecular weight of solute | g/mol |
| % | Percent weight/volume | g/100 mL |
| purity | Fractional purity of the solid | — |
How stock solution is calculated
- For a molar stock, multiply target molarity by final volume in litres to get moles, then by molecular weight to get grams of pure solute.
- For a percent stock, multiply % w/v by the final volume in mL and divide by 100 to get grams of pure solute.
- Divide the pure-solute mass by the purity fraction to get the mass of the actual solid to weigh.
- Dissolve in about 80% of the final volume, then bring the solution up to the exact final volume and mix.
Worked laboratory examples
Example 1 — 500 mL of 1 M NaCl
Given: M = 1 mol/L · V = 500 mL = 0.5 L · MW = 58.44 g/mol · purity 100%
- moles = 1 × 0.5 = 0.5 mol
- mass = 0.5 × 58.44 = 29.22 g
Result: weigh 29.22 g NaCl, dissolve and make up to 500 mL
Example 2 — 1 L of 1 M from a 95% pure solid
Given: M = 1 mol/L · V = 1 L · MW = 100 g/mol · purity 95%
- pure mass = 1 × 1 × 100 = 100 g
- as-weighed mass = 100 / 0.95 = 105.26 g
Result: weigh 105.26 g of the 95% solid — 5.26 g more than the pure figure
Example 3 — 200 mL of 10% w/v glucose
Given: % w/v = 10 · V = 200 mL
- mass = 10 × 200 / 100 = 20 g
Result: weigh 20 g glucose, dissolve and make up to 200 mL
Want the reasoning behind the numbers? ReadWhat Is Molarity?.
How to interpret the result
The mass on the primary card is what goes on the balance. For a molar stock the mole count tells you the chemical scale of what you are making — 0.5 mol in 500 mL is the same 1 M whether you make 500 mL or 5 L, only the mass changes. For a percent stock there are no meaningful moles without a molecular weight, so that card is intentionally blank.
The purity correction is the part most often skipped and most often wrong. A reagent labelled 95% means 5% of what you weigh is not your solute, so you need about 5% more solid to hit the target. The calculator reports the extra mass explicitly so you can see how much the correction is actually contributing — sometimes negligible, sometimes the difference between a working and a failed buffer.
Assumptions
- The solute dissolves completely and does not change the final volume beyond what you measure to the mark.
- Purity refers to the mass fraction of the target species, and the impurity is inert (does not contribute the solute or affect volume).
- The molecular weight matches the form on the bottle, including any waters of hydration.
Limitations
- Solubility limits are not checked — a mathematically valid stock may be impossible to dissolve at that concentration or temperature.
- Percent is interpreted as weight/volume (g per 100 mL); weight/weight percent, which needs density, is not modelled.
- Stability, sterilisation and storage are outside the calculation; some stocks must be made fresh or filtered rather than autoclaved.
Common mistakes
- Adding the solute to the full final volume of water instead of dissolving in less and making up to the mark — the solute itself displaces volume, so the concentration ends up too low.
- Ignoring purity and weighing the pure-solute mass from a 90% or 95% bottle.
- Using the anhydrous molecular weight for a hydrated salt. Check whether the label says, for example, ·H2O or ·5H2O.
- Confusing % w/v with % v/v or % w/w. A 10% w/v solution is 10 g of solid per 100 mL of final solution.
FAQ
How do I make a stock solution?
What is the difference between a molar and a percent stock?
How does purity affect the mass I weigh?
Why dissolve in less solvent first?
How concentrated should a stock be?
How do I make a working solution from the stock?
Scientific Sources
- Promega Biomath — Molarity Calculator · Tier 2 source
- NEB NEBioCalculator · Tier 2 source
- NIST Chemistry WebBook · Tier 1 source
External links open in a new tab and carry rel="nofollow". We cite primary references (NIST, NIH/NCBI, peer-reviewed papers) and established manufacturer technical documentation (NEB, Thermo Fisher, Promega, IDT). Calculated results are estimates; your protocol and manufacturer guidance take precedence.
Last reviewed: 2026-09 · Method version: 1.0