Write the formula for finding an object's force in Newtons, F=ma, at the top of the page. Since you are trying to find the mass (m), you will need to isolate m on one side of the equation.
Divide by a on both sides of the equation, and cancel out the a on the right side. This will leave you with F/a = m, which you can switch to m = F/a to get the variable you're looking for on the left.
Plug the numbers you want to convert to mass into the new equation, m = F/a. As an example, we will use an object with a force of 10 N and an acceleration of 2 meters per second squared.
Divide the newtons by the rate of acceleration, which will give you the mass of the object. The mass will be in kilograms, because a single newton represents the amount of force needed to move one kilogram one meter. For our example, we will divide 10 N by 2 m/s/s, which give us a mass of 5 kg.