How to Measure Chimney Height

Q: How do I determine flue length to see how much liner I need?
-Kate Pavelle

Your question brings to mind a story I once read in a book about super-intelligent children:

On test day in Science class, the students were given the following question:

"How can you use a barometer to determine the height of a building?"

One student's answer:

Off-hand, I can think of four ways.
1) The obvious answer; compare the barometric pressure readings at the bottom and top of the building.
2) Measure the height of the barometer, and walk it up the side of the building (barometer height x repetitions = building height).
3) Toss the barometer off the top of the building, and time its descent until it hits the ground (falling objects accelerate at the rate 32 ft/second²).
4) Go to the owner of the building, and say, "Hey, I'll give you this dandy barometer if you tell me how tall your building is."

If you don't want to use a barometer, the most accurate way to determine flue height is to drop a measuring tape down the flue from the top of the chimney. If you don't have a long enough tape measure, drop a rope, and tie a knot at the point where it exits the chimney. Back on the ground, measure from the end of the rope to the knot, in sections if necessary.

If the chimney is made of bricks or cinder blocks and on an outside wall where you can see its entire length, you can simply measure one course (the height of one brick or cinder block plus one mortar line), and count courses.

On a sunny day, pound a stick or dowel into the ground. Measure the length of the stick above the ground, then measure its shadow. (For example, the stick might be 3 feet tall; its shadow might be 2 feet long.) Having determined that a 3-foot stick casts a 2-foot shadow, measure the shadow cast by the chimney. If the chimney's shadow is 10 feet long, you can use a little algebra to figure out the height of the chimney: If 2/3 = 10/x, then 2x =10 * 3 or 2x = 30, so x = 15; the chimney is 15 feet tall.

Finally, the yardstick technique (this one is uncannily accurate):
1) Make a fist, and hold your arm straight out in front of your body, parallel to the ground, with your thumb pointing up.
2) With a yardstick, measure the distance between your eyes and the tip of your thumb. We'll call this distance E-T (Eye-Thumb)
3) Assume the same posture as in step 1, but grasp the yardstick in your fist and hold it vertically at arm's length so that E-T inches extend above your thumb.
4) Facing the chimney, walk backwards until the chimney appears to be the same height as E-T (the portion of the yardstick that extends above your thumb).
5) Measure the distance between you and the chimney. This will equal the chimney height.

The last three methods will give you an approximate height. If you measured all the way to the bottom of the fireplace, you'll need to subtract the height of the flue collar on the hearth stove or insert, and add the 4-5" that need to protrude out the top of the chimney.

Since our liner kits only come in 5' increments, and can be trimmed down but not stretched, it might be prudent to buy the next-longer kit than these techniques indicate.