Molarity Calculator
Calculate how much compound to weigh, determine solution concentration, or prepare dilutions from an existing stock solution. This molarity calculator helps scientists prepare reagents, standards, buffers, and assay stocks while automatically handling common laboratory unit conversions.
Calculator
Choose the workflow that matches what you are doing at the bench. Only the inputs needed for that workflow will appear.
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ELN-Friendly Method Summary
Formula and Calculation Logic
Molarity is moles of solute per liter of final solution. The calculator converts each input to base units before applying the selected rearrangement.
Mass is normalized to grams, volume to liters, and concentration to mol/L. Stock and target concentrations are normalized before dilution calculations. Preparation overage scales final volume and required dry-compound mass together.
Example Workflow
Prepare 10 mL of a 100 mM solution using a compound with a molecular weight of 500 g/mol.
Select "Calculate Required Solute Mass" and enter 100 mM, 10 mL, and 500 g/mol.
The calculator determines that 500 mg of compound is required.
Weigh 500 mg, dissolve in less than 10 mL solvent, then bring the solution to a final volume of 10 mL.
Common Mistakes
Each step changes concentration by 1,000-fold. Confirm both the number and selected unit.
Free base, salt, hydrate, and other forms can have different molecular weights.
Dissolve the solute below the target volume, then bring the solution to final volume.
Scale the preparation or make an intermediate stock when the calculated quantity is too small to measure reliably.
A 10 mM stock diluted 1:10 produces a 1 mM working solution. Always confirm whether a protocol refers to stock concentration or final concentration.
A calculated concentration may be chemically correct but still impossible to dissolve. Confirm solubility, solvent compatibility, temperature requirements, and compound stability separately.
The calculator assumes the entered mass is fully active compound unless a separate purity correction is applied.
mg/mL, µg/mL, M, mM, and µM are not interchangeable without molecular weight. Confirm whether the protocol expects mass concentration or molar concentration before preparing the solution.
Frequently Asked Questions
How do I calculate grams needed for a molar solution?
Multiply the target molarity (mol/L) by the final solution volume (L) and molecular weight (g/mol). The calculator performs these conversions automatically and supports common laboratory units such as M, mM, µM, L, mL, and µL.
Should final volume include the solute?
Yes. Dissolve the solute in less than the target solvent volume, then adjust the complete solution to the stated final volume.
Which molecular weight should I use?
Use the molecular weight of the exact material being weighed. Free-base compounds, salts, hydrates, and other forms of the same molecule may have different molecular weights and therefore require different masses to achieve the same target concentration.
Does the calculator account for reagent purity?
No. Correct the weighed mass separately when assay value or purity is below 100% and the protocol requires that correction.
How do I dilute an existing stock?
Select Dilute a stock solution, then enter the stock concentration, lower target concentration, and desired final volume. The calculator reports stock and diluent volumes.
Does molarity depend on density?
Not for a final volume entered directly. Density is needed when converting a liquid reagent's mass or volume into moles, which this calculator does not perform.
Does the calculator verify solubility or pH?
No. The calculator determines the theoretical amount required to achieve a target concentration. Solubility limits, precipitation risk, pH effects, stability, density corrections, and solvent compatibility must be evaluated separately.
What is the difference between M, mM, and µM?
M (molar), mM (millimolar), and µM (micromolar) differ by factors of 1,000. One molar equals 1,000 millimolar, and one millimolar equals 1,000 micromolar. Unit-selection mistakes are one of the most common causes of concentration errors during solution preparation.