How do I calulcate the amount of concentrated stock solution to add to get the correct dilution?

For example, how much loading buffer (6X) do I need for my PCR reaction with a volume of 25 μl? What is the general way to calculate it?

For any question regarding volume and concentration, you should consider the equation $$C_1V_1=C_2V_2$$. We can use it to derive a general equation for the addition of concentrated stock solutions. We want to find the volume of stock solution ($$V_1$$) to add so that its final concentration is 1X. Here's what we know:

• The initial concentration of stock solution, $$C_1=X$$
• The final concentration of the solution, $$C_2=1$$
• The volume of solution we are diluting the stock into, $$V_s$$

So the equation becomes:

$$XV_1=V_2$$

We don't know $$V_2$$, but we do that $$V_2=V_1+V_s$$. Subbing that in:

$$XV_1=V_1+V_s$$

Solving for $$V_1$$:

$$XV_1-V_1=V_s$$ $$V_1(X-1)=V_s$$ $$V_1=\frac{V_s}{X-1}$$

Then you simply need to sub in your values:

$$V_1=\frac{25μL}{6-1}$$ $$V_1=5μL$$