Experience

Killing monsters and fulfilling certain tasks will grant players experience (also known as exp, xp, or experience points), which they accumulate to increase skill levels. Training skills generically is the typical way to gain experience, however quests often reward some.

To reach level 99 in a skill, the player needs to reach 13,034,431 XP. Half of that amount is nearly enough for level 92, and a quarter of it barely reaches level 85. To reach level 99 in all 18 skills, the player will have accumulated 234,619,758 experience.

Hits
The Hits skill is a special case in that all accounts start with level 10 Hits and 1,000 XP (after character classes were removed). This isn't enough for level 10 in other skills, however. When was released, accounts that had not reached the full 1,154 Hits XP for true level 10 had their Hitpoints level in  set to the correct value for 1,000 XP  level 9.

Experience table
The following table shows the relationship between skill level, the experience required for that level, and the difference in experience from the previous level:

Maximum experience in a single skill
Unlike, skills in do not have an experience cap at 200 million. Instead, it is not possible to exceed 536,870,911 ($$\frac{2^{31} - 1}{4}$$) experience in a single skill because of data type limits.

Passing the maximum XP results in an integer overflow, and the displayed value will wrap around to a very large negative number. It is possible to continue gaining experience afterwards and return to a positive number. Doing so does not cause the level to reset to level 1. Only one character is currently known to have reached the negative experience point, doing so in Thieving, however, the player admitted to botting to do it.

The reason for the limit being a quarter of the usual maximum for 32-bit singed integers (2,147,483,647) is because of precision points. The number is divided by four before being displayed in order to allow for a limit set of possible experience values below 0 (0.25, 0.5, and 0.75). For example, killing a chicken gives 6.5 Hits XP, but the number beyond the decimal point is not displayed on screen.