The Votta’s equation is usually used for constant-force spiral spring. But the calculated result is not consistent with experiment. One of the main factor comes from the value of Young’s modulus. Generally, Young’s modulus measured by tensile test, does not quite correspond to the value by bending test. We measure the Young’s modulus by bending once more to verify the validity of equations. Furthermore, we point out some logical mistakes in Votta’s equation.We formulate the rigorous equation considering variable load, and then simplify this assuming constant load for practical design.