You'll want to change the strings on your ukulele if one of these apply:

The tuning accuracy down the neck differs.
The strings buzz when they didn't before.
You run your fingers under the strings and can feel dents where the frets have damaged the strings.
The string has snapped.
The strings are peeling.;
,
If you have a Concert-sized ukulele, you should buy a 23" string.
If you have a Tenor ukulele, you should buy a 26" string.
If you have a Baritone ukulele, you should buy a 30" string.
 There are many different types of materials used to make ukulele strings.

Nylon is often used because of its round and warm sound which mimics the sound of gut strings. It also provides tuning stability and resistance to humidity.
Fluorocarbon strings provide an even fuller sound than nylon string and reacts less to temperature change.
Titanium strings offer strength and durability with a louder and brighter sound.
Steel strings can be used on the ukulele, but it not recommended as it may cause damage to the bridge and neck of the instrument.
 Before tying the string to the ukulele, you need to create a knot in the string.

First, make a simple knot in the string.
Make a loop.
Overlap the loop on the knot, pulling it downward.
Tighten the loop on the knot.
 The number depends on how your bridge was made. Larger tie blocks can take up to three loops on the thinner strings while narrow tie blocks usually take only two. The fatter strings on the ukulele usually only wrap around once or twice., Then pull the string taut and set it into the notch in the nut., You can check out How to Tune a Ukulele for how to do that.
Decide when to change the strings.
Pick the right length:

If you have a Soprano (Standard) ukulele, you should buy a 21" string.
Consider your material.
Knot the string.
Wind the string knot around itself one to three times.
Pull the end of the string toward the head of the ukulele and pull it through the metal pin of the tuning peg.
Tune your ukulele.
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