If you ever find yourself without a guitar tuner and you're itching to play, I've got some quick and dirty tricks to help you tune your guitar by ear. Of course, using a tuner is always the best practice, but sometimes we're caught in situations where we don't have one, like when it dies on us or when we're out in the woods with no equipment around. So, let's get started!
First, start with your sixth string and get it locked in with another instrument that is in tune, like a keyboard or something similar. Some people carry a tuning fork in their case for this purpose. Once you're happy with your E, tune the rest of the guitar by ear by going up through the strings. You can match it up with an A here at the fifth fret.
If you're having a hard time hearing that reference pitch on the sixth string (fifth fret) and need to adjust your tuning with that same hand, don't worry. You can still use a little trick by playing harmonics. Rest your finger directly over the 5th fret, touching the string but not pressing it all the way down. When you pick the string and take your hand off, you'll still get a harmonic ringing out. This gives you a reference note you can compare others to.
You can do the exact same trick at the seventh fret of the next string, the fifth string in this case. Afterward, tune the next pair of strings and keep on bringing that pitch up all the way through the strings, using the fifth fret on the tuned string, and the seventh fret on the string you're tuning.
Pro Tip: If you're not sure if one of the strings is already tune, drop the tuning peg down quickly on the string you are tuning, and then bring it back up. Your ear will have a much easier time telling what's in tune and what's not this way.
When you get to the second and third strings, you can't use the harmonics anymore, so you have to use the fourth fret on the third string, which is a B, and then the open second string, which is also a B. Just try to match them up as best as you can. The harmonics will work again for strings one and two.
It's often useful to go back through for a second pass to ensure that everything is tuned correctly. This technique is a good way to at least get something out of your guitar when you don't have an electronic tuner. While using a tuner is always better, this method is a quick and dirty way to tune your guitar by ear.