The various increments of standard wire sizes differ by only about 0.010 inch, so calipers or a micrometer would be the tool to use to be certain of the stepped size you have, or else a trustworthy ruler marked in tenths of an inch to use the measure-10-turns-and-divide-by-10 trick. The most common wire sizes in the US are 0.207", 0.218", 0.225", 0.234", 0.243", 0.250", and 0.262".
All measurements should be in feet and inches. Step 1, measure across the existing door or desired space for the width, then up and down for the height. The rough opening of your space should be the same size as the door. Step 2, measure the sideroom, which is the space beside your door. Measure the width of the left side, and then the width of the right side. Step 3, measure the space above the door, which is called the headroom. Measure the height of the distance between the top of the door opening and the ceiling. Step 4, measure the ceiling, which is called the backroom. You’ll need to measure the distance of the garage door opening toward the back wall of your garage. You should have 6 total measurements in all once you’ve finished measuring the space. Keep in mind that having an automatic garage door opener installed might call for additional backroom or ceiling space.
The optician's trick: The serviceman looks over your door with lots of scowling, chin-scratching, and tsk-tsking. You ask, "how much?" He replies with the fair price. If you don't flinch at that price, he says, "for the parts", while quoting a large additional cost for the labor. If you still don't flinch, he adds, "each," while pointing back and forth to your pair of springs. (I hope none of you service people are reading this!) I call this the "optician's trick" after the old vaudeville joke about lenses, frames, and left/right.
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Right-hand versus left-hand winding: Springs are chiral, that is, wound or "laid" in either a left- or right-hand orientation. This is a critical property of their design and specification; you cannot substitute a left for a right or vice versa. If you were to grasp the spring in your hand, and if your right hand orients the tips of your fingers like the ends of the coiled wire when your thumb points "out" of the core of the spring, then you have a right-hand spring; likewise left (which end you grasp does not matter). (This also happens to match the "right hand rule" of magnetic polarity, if you happen to be knowledgeable in such esoteric subjects.) Another way to identify the winding is to examine the spring vertically in front of you; if the coils facing you rise going to the right, it is right-hand (thus you can remember, "rise to the right is right-hand"), and likewise left indicates left-hand. Another way is to view the coil axially; a right-hand spring winds in a clockwise direction as it recedes away, and a left-hand spring counter-clockwise. Yet another way, not so easy to remember, is to hold the spring vertically and compare the coil shape to the letter "Z" (indicates right-hand lay) or the letter "S" (indicates left-hand lay).

Uncentered center bearing plate: The center bearing plate need not actually be in the center. It doesn't much matter where it is, since the purpose of the bracket is to anchor the spring ends. This anchoring must be secure, since all the torsion is held together at that point. On a stud-framed wall, this bracket may be placed over the stud closest to the center rather than exactly at the center of the door opening.
Most situations allow you to replace spring(s) without removing the assembly from the wall, if there is enough clearance in the surrounding garage structure at the ends of shafts. By unbolting the end bearing plates and removing the drums, you can run the springs down to the ends of the shaft to remove and replace the springs. This avoids the balancing act of holding a long, wobbly, heavy shaft while climbing up and down a ladder. This is how the professionals get the job done in a few minutes.

One can overwind the springs slightly, up to about 8 turns on a standard residential door (that is, 1/2 or 3/4 extra turns), to compensate for undersized or fatigued springs, or increased door weight from painting or humidity, but this results in more stress on the springs and therefore decreased lifetime. If the door is too heavy for that slight tweak, then different springs are needed.
Security Lights: Most newer garage door openers have at least two bright light bulbs, as well as lights activated by motion. On the other hand, some older or cheaper openers are limited to a single light bulb. This doesn't help much with a garage darkened by shadows, leaving you unsure if the rustling in the corner is your cat … or a skunk that snuck into the garage.
Note the left winding cone with red spray paint. This shpritz of paint is applied to create fear and doubt in the mind of the do-it-yourselfer. Sometimes it is a color code for the wire size (using a DASMA standard, red indicating 0.2253 inch diameter wire). Sometimes it indicates the winding direction: red may indicate right-hand winding, but don't rely on that; do you own independent analysis. Sometimes it is a manufacturer's private code for another dimension than wire size. This color code is for the installer's information when the spring is new; I would not depend on interpreting the color code properly on an old spring, since one can't be certain of a correct interpretation without documentation from the original supplier.
Dodging a falling door:: Reversing this equation gives us x=gt^2/2 as the fallen distance x for a given time t. How much time would you have to dodge a falling door if the spring were to suddenly break at the top of travel? Let us assume you are 5.5 feet tall, so the door will hit your head after falling 2 feet from its 7.5 foot fully-raised height. This 2-foot fall takes sqrt(2*2/32.2) = 0.35 seconds (350 milliseconds). The quickest human response time is about 200 milliseconds, so even if you are alert to the hazard, this leaves you only about 150 milliseconds to accelerate and move your noggin out of the way. If you are an Olympic gold medalist in the 100 meter dash, you can accelerate (horizontally) about 10 feet/second^2, and your 150 milliseconds of wide-eyed panic will move you all of 10*0.15^2/2 = 0.11 foot = 1.35 inch.

The winding technique is simply to (un)wind as far as one rod will go, where it is pressed against the top of the door, or nearly so, by the unwinding torsion. You insert the other rod in the next socket, remove the first rod, and continue. At any point you can stop and rest by leaving the active rod pressed against the door, where it will be held by the unwinding force. I would make a quarter-turn increment that way, and let go for a moment to collect my attention for the next increment, almost in a quiet, meditative alertness. While you can go from one quarter-turn and rod-swap to the next continually without letting go, working fast against the steady tension seemed to invite a kind of shakiness in my arms that was a bit unsettling. It isn't that there is much physical exertion, it is more that the tension is unrelenting, like peering over a precipice.
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