What is a PCR master mix?

A master mix is a single pooled batch of the reagents shared by every reaction, such as polymerase mix, primers, and water. Aliquoting one mix into each tube cuts pipetting steps, improves consistency, and reduces well-to-well variation.

Every pipetting step loses a little liquid to tips and walls, so a mix made for exactly the number of tubes runs short on the last one. An overage of 5 to 10 percent makes slightly more than needed so each reaction gets its full volume.

Should template DNA go in the master mix?

Usually no. Template differs from sample to sample, so it is added to each tube individually rather than the shared mix. Mark template as a per-tube component and the calculator excludes it from the pooled volume while still counting it toward the reaction total.

How is the water volume found?

Water fills whatever is left after the defined components are subtracted from the total reaction volume. If your listed components already exceed the total, the water comes out negative and you must lower a component or raise the reaction volume.

Can I use this for qPCR?

Yes. List your 2x qPCR mix, primers, probe, and water as in-mix components and your template as per-tube, set the reaction count and overage, and read the scaled volumes. The same arithmetic applies to endpoint PCR and qPCR alike.

PCR Master Mix Calculator

Setting up dozens of identical PCR reactions by hand invites arithmetic slips. This calculator scales each reagent from a per-reaction recipe to any number of reactions, adds a pipetting overage, and fills water to the reaction total — while keeping per-tube reagents like template DNA out of the shared mix.

How it works

The mix is scaled to slightly more than the reactions you need, then water back-fills the per-reaction total:

effective reactions = reactions × (1 + overage% / 100)
component mix volume = per-reaction volume × effective reactions
water per reaction   = total reaction volume − sum of component volumes

Components ticked as in-mix are pooled and multiplied by the effective reaction count; per-tube components (template DNA) are listed for reference but excluded from the pooled volume, since they are pipetted into each tube individually.

Tips and example

For 10 reactions at a 10 percent overage the mix is made for 11 effective reactions. With a 25 µL reaction holding 12.5 µL of 2x mix, 1 µL each primer, and 2 µL of per-tube template, water fills the remaining 8.5 µL. Always vortex and briefly spin the master mix before aliquoting so the enzyme and salts are evenly distributed across every tube.