Measuring your torso length correctly is critical so I can build you a properly fitting backpack. You will need a helper to do this accurately and get them to repeat the measurement at least 3 times to be sure. Get this wrong and your pack will not fit correctly!
Measure from the C7 vertebra - tilt your head forward and locate the bony lump at the base of the neck, this is the C7 vertebra, and then raise your head to its normal position - using a tape get your helper to measure directly from the C7 down to the iliac crest (hip shelf) which is the first hard object you feel running your fingers down your sides after your rib cage. You should position both hands on the iliac crest so that the thumbs are reaching behind and pointing inwards to your spine. Get your helper to make sure they are level and measure to the point where the tape crosses an imaginary line between the bottom line of the thumbs. This is your Torso Length.
If your measurements differ by more than 1cm then recheck you have identified the C7 vertebra and iliac crest correctly and repeat. Keep going until you are consistently getting measurements within 1cm and then always go with the largest number.
When building a pack I will assume the upper edge of the hipbelt will sit 2.5cm above your iliac crest, this is normally regarded as the ideal position for a hipbelt. In this position, with a lightly loaded pack, the shoulder straps will normally be a little higher than the tops of your shoulders. We are, however, all different and you may prefer your hipbelt elsewhere. Where your hipbelt sits obviously alters the pack back length so it’s important to let me know if you like your hipbelt to sit higher or lower than 2.5cm above your iliac crest. For instance some female hikers prefer the hipbelt to sit much higher than this.
When measuring your belt size use a string or cloth tape measure pulled tightly around the spot on your body that you want the belt to ride. Usually this will be around your pelvic bone and NOT your waist. You should do this measurement wearing the least amount of clothes you would ever hike in. Do not use your trouser size!
Small (66cm of padding) Adjusts 72-104cm
Medium (76cm of padding) Adjusts 82-114cm
Large (86cm of padding) Adjusts 92-124cm
X Large (96cm of padding) Adjusts 102-134cm
A Medium belt has 76 cm of padding, and 38 cm of webbing. It will adjust from 82-114 cm. This adjustment allows space in front of the padding for the buckle. If you are within the Adjusts range then the belt should fit fine. Even if you are near the lower limit there is no need to size down. The exception to this is if you are near the lower limit and expect to drop weight, in that case size down so that you have room for adjustment.
My measurement is 83cm so I could fit a Small or Medium belt. When choosing between sizes pick the largest that will fit you. The medium belt fits me perfectly and surrounds my hips in padding while still leaving enough room for the buckle. If I had expected to drop weight then the small could have been a better choice.