When taking measurements for a pair of trousers, it’s very important that you are taking each measurement from the same height. In the 1860s, trousers were worn at the natural waist, or about the level of the belly button.
If you have trousers that fit properly for the period, please take your measurements while wearing those. If not, tie a string or ribbon securely around the waist at this height to reference your measurements from.
1) Side Seam
Or length of side measured from the top to the bottom of the trousers, starting from the top of the side seam at the hollowest part of the waist, and including the waistband. This measure should be taken tightly, and it is the leg itself, not the trousers, which is to be measured.
When taking the first three measures, it is easiest to measure to the heel of the shoe, so as to be consistent. Then make a deduction based upon how you want the trousers to break. Some like the trouser legs to reach exactly to the foot, while others like to have a break or even folds at the ankle.
2) Front Length
Measured from the top of front, including the waistband, to the bottom of the inner leg seam. In thin waists, the measure will generally be less than the length of side seam; it will be longer for stout men on account of the protuberance of the stomach, and by the difference it presents with measure of Side, it indicates the proper degree of slope to be given to the front of the waist. To take it correctly, the leg should be a trifle advanced as shown on fig. 1, and for very stout men, the measure should be kept close to the leg of trousers, just below the fork.
3) Leg Seam or Inseam
Measure from the crutch down to the bottom of the heel as you did with the previous measurements.
To take this measure, hold the tape close up in the fork of the trousers, letting the tape fall naturally to the foot at the bottom of leg seam. We may observe that when using the looped tape, it should not be pushed up too tight, or the leg seam measure would become too long, and the trousers would be too long in the legs.
Measure the waist at the top of the waistband, or along the ribbon if you used that method. The measurement should be taken rather tightly.
5) Seat or Hips
This is the size round the seat at the most prominent part, just over the hip joint. This measure often gives the same figures as the breast measure of the man, but is more frequently a little larger.