If you cannot find any information on the bands you have, and if you are keen, you can measure the resistance of each band using a simple formula based upon Hooke's Law, which is expressed as follows:
F = k . x
where F is the force applied to the band; x is its extension; and k is the spring constant, which is essentially a measure of the resistance of the band as we might normally think of it. That is, k is how much it stretches when we apply a certain force to it, or alternatively, how much force we have to apply to it in order to make it stretch a certain distance.
Rearranging our formula from Hooke's Law in terms of k, you have:
k = F ÷ x
In order to measure k, for each band do the following:
- Tie the band to the handle of a bucket;
- Suspend the bucket from (tie the other end to) a doorway or other appropriately strong overhead fixture;
- Measure the height from the floor of the empty bucket;
- Place an object of known mass in the bucket;
- And measure the height from the floor of the laden bucket.
For the lighter bands, you may have to double the band over, or loop it around the bucket and up again. In such cases, you perform the measurement as above, then simply halve the figure after you have made the calculation.
Thus, if you were to place a gallon-bottle of water (or milk, or juice) in the bucket, which has a mass of 8.34 pounds, and the bucket thereby dropped 5 inches from its unladen height, you would have...
k = 8.34 ÷ 5 = 1.677 lbs per inch ≈ 20 lbs per foot
Note that it does not matter which units, metric or imperial, you choose to use, since k is simply a ratio—a relationship between two variables.
I hope that is helpful.