Fourths tuning uses the pitches E2-A2-D3-G3-C4-F4 from low to high. The bottom four strings (E-A-D-G) are tuned exactly like standard tuning. You only change the B string up a half step to C, and the high E string up a half step to F.
How to Tune
Start from standard tuning. The low four strings are already correct. For the 2nd string (normally B), tune up one half step to C. For the 1st string (normally high E), tune up one half step to F. You can verify by fretting the G string at fret 5 - it should match the open C string. Then fret the C string at fret 5 - it should match the open F string.
A clip-on chromatic tuner is the easiest option. Most tuner apps work fine too. Just make sure you are reading the correct note name - C4 for the 2nd string, F4 for the 1st.
String Tension
Tuning the B string up to C and the high E up to F increases tension on those two strings. With a standard set (e.g., 10-46), the high F string will feel noticeably tighter. This is playable but not ideal for long sessions or heavy bending.
Most fourths tuning players use a lighter gauge on the top two strings. A common approach: use a standard 10-46 set but swap the high E for a .009. Some players use custom sets like 10-13-17-26-36-46 to balance tension across all six strings.
The Half-Step-Down Alternative
Another popular option is tuning the entire guitar down a half step: Eb-Ab-Db-Gb-B-E. This keeps all six strings in perfect fourths while avoiding the increased tension on the top two strings. The B and E strings stay at their standard tuning pitches - you are only lowering the bottom four.
The downside: everything sounds a half step lower than concert pitch, and fret numbers no longer correspond to the same note names. If you play with other musicians, you will need to transpose. For solo practice and learning, either approach works. This app assumes standard fourths (E-A-D-G-C-F) throughout.
The open string notes (E-A-D-G-C-F) are highlighted on the fretboard above. To verify your tuning, fret each string at the 5th fret and check that it matches the open note of the next higher string.
Verification Checklist
Play the low E string at fret 5 - it should match the open A string. Play A at fret 5 - it matches open D. Play D at fret 5 - it matches open G. Play G at fret 5 - it matches open C. Play C at fret 5 - it matches open F. If any pair does not match, retune that string.