For greater accuracy, take measurements with no figures, and with those pants that sit on you well. Lay the jeans on a flat surface and attach a ruler or measuring tape to the belt so that it is held on the button. Measure the distance between the two extreme points of the belt. Multiply the result by two, if the line has inch division. If you measured the waist size in inches, then multiply the result by two and divide by 2, 54 (that is how much centimeters in one inch). Round the value obtained in a big way, if you want the jeans were loose, or less if you prefer-fitting model.
There is another method for determining the amount of belt, however, it is not very accurate. Just subtract from its normal size 16 units. For example, if you wear a dress size 46, the size of the belt is equal to 30 inches.
To determine the second parameter - length - measure the inseam jeans. This is best done on the trousers worn by you, preferably with an assistant. Determine the length of the pant leg from the groin to the level that, in your opinion, the most preferred. Do not forget to share the result by 2, 54, if you measure the length in centimeters.
Keep in mind that after washing jeans sit in length is much stronger than it is wide. This should be considered if you get pants from untreated tissue. It shrinks 10-15%, but the treated fabric lasts not more than 2%.