Four numbers behind every map
Every difficulty in a beatmap set comes with four settings chosen by the mapper, each on a scale from 0 to 10: AR (approach rate), OD (overall difficulty), CS (circle size) and HP (HP drain). Together with the song's speed and the pattern itself, they decide how hard a map feels. The osu.net player reads them from the beatmap file but does not print them on screen, so it helps to know what each one does and how to recognize it by feel.
Below are the exact formulas the player uses. Times are in milliseconds, and sizes are in osu! pixels on the 512 by 384 playfield, which is scaled to fit your window.
AR: approach rate
AR controls how early an object appears before you have to hit it. A circle fades in, its approach circle shrinks, and the time from first appearance to the hit is the approach time. Below AR 5 the approach time is 1800 − 120 × AR; from AR 5 up it is 1950 − 150 × AR.
- AR 3: 1,440 ms. Calm and easy to read, but objects pile up on screen in dense parts.
- AR 5: 1,200 ms. A typical middle value.
- AR 8: 750 ms. Objects appear quickly; you need to react and trust the rhythm.
- AR 10: 450 ms. Almost no warning. Reading becomes the main challenge.
Low AR is not automatically easy. On a low-AR map many objects are visible at once, and it can be hard to tell which one comes next. High AR shows fewer objects, but you get less time to move.
OD: overall difficulty
OD sets the timing windows, meaning how far from the exact beat you can hit and still get each judgement. In this player a hit counts as a 300 within 80 − 6 × OD milliseconds of the beat, a 100 within 140 − 8 × OD, and a 50 within 200 − 10 × OD. Clicks outside the 50 window are ignored.
- OD 2: ±68 ms for a 300, ±124 for a 100, ±180 for a 50.
- OD 5: ±50, ±100 and ±150.
- OD 8: ±32, ±76 and ±120.
- OD 10: ±20, ±60 and ±100.
Higher OD means accuracy drops quickly when your timing drifts. OD also sets how fast you must turn spinners: the higher it is, the more rotations a spinner needs.
CS: circle size
CS sets the size of circles and sliders. The radius is (109 − 9 × CS) ÷ 2 osu! pixels, so higher CS means smaller circles. At CS 2 the radius is 45.5 pixels, at CS 4 it is 36.5, at CS 5 it is 32 and at CS 7 only 23. You have to click inside the circle, so small circles demand precise aim, especially on jumps. Bigger circles also overlap more, which can make dense patterns harder to read.
HP: HP drain
HP controls how much the HP bar reacts to your hits. In this player the bar never drains on its own; it changes only when you are judged. A miss removes 2 percent of the bar for every point of HP, so at HP 5 each miss costs 10 percent. A 300 gives back 10.2 − HP percent, a 100 gives back 8 − HP percent, and a 50 gives 4 − HP percent, which is negative above HP 4. At HP 8 a 100 restores nothing and a 50 costs 4 percent.
If the bar runs empty you keep playing until the end, but the result is graded F. High HP maps therefore punish streaks of mistakes, while low HP maps forgive almost anything.
Star rating
The star rating is a single number that sums up how hard a difficulty is. The player takes it from the mirror's data and shows it as a row of ten stars in the difficulty pop-up and as a colored ring on each map card. The ring colors follow these bands: easy below 2 stars, normal below 2.7, hard below 4, insane below 5.3, expert below 6.5, and expert-plus from 6.5 up. Mods do not change the stars shown.
How mods change the numbers
Easy halves all four values. Hard Rock multiplies AR, OD and HP by 1.4 and CS by 1.3, never going above 10. Nightcore and Daycore do not touch the numbers, but because the whole song plays at 1.5 or 0.75 times speed, every time-based value shrinks or stretches with it. An AR 8 map under Nightcore gives you 500 real milliseconds instead of 750, and its timing windows shrink by a third. Hidden leaves the numbers alone and removes approach circles instead. The mods guide has the details.
| Value | Controls | Formula in this player | Higher means | Easy / Hard Rock |
|---|---|---|---|---|
| AR | How early objects appear | 1800 − 120 × AR ms below AR 5, else 1950 − 150 × AR ms | Less time to react | × 0.5 / × 1.4 |
| OD | Timing windows, spinner speed | 300: 80 − 6 × OD, 100: 140 − 8 × OD, 50: 200 − 10 × OD ms | Stricter timing | × 0.5 / × 1.4 |
| CS | Circle size | Radius (109 − 9 × CS) ÷ 2 osu! pixels | Smaller circles | × 0.5 / × 1.3 |
| HP | HP lost and regained per hit | Miss: −2% × HP; 300: +(10.2 − HP)% | Harsher HP bar | × 0.5 / × 1.4 |
Picking maps for your level
Difficulty names such as "Normal", "Hard" or "Insane" are chosen by each mapper and are not standardized, so trust the stars and ring color more than the name. A sensible path looks like this:
- Start with easy rings, below 2 stars. Aim to finish with a B or better.
- Move up when you pass most maps at your level with an A. Step up by about half a star at a time.
- Check the BPM and length on the map card. A fast song at a low star rating still asks for quick taps, and a long map tests stamina.
- If one map feels too hard, use Easy or Daycore to learn it, then play it normally.
- If everything feels too easy, try Hard Rock on maps you already know before moving to a higher star rating.
Each value also has its own feel. If you keep missing because circles appear too suddenly, look for lower-AR maps. If your accuracy is low even though you hit everything, the OD may be high for you. The tips page and the glossary cover more of the vocabulary.