In the US, section 4.8, Ramps of the ADA has this to say:
4.8.2* Slope and Rise. The least possible slope shall be used for any ramp. The maximum slope of a ramp in new construction shall be 1:12. The maximum rise for any run shall be 30 in (760 mm) (see Fig. 16). Curb ramps and ramps to be constructed on existing sites or in
existing buildings or facilities may have slopes and rises as allowed in 4.1.6(3)(a) if space limitations prohibit the use of a 1:12 slope or less. Appendix Note
4.8.3 Clear Width. The minimum clear width of a ramp shall be 36 in (915 mm).
with the Appendix Note:
A4.8.2 Slope and Rise. Ramp slopes between 1:16 and 1:20 are preferred. The ability to manage an incline is related to both its slope and its length. Wheelchair users with disabilities affecting their arms or with low stamina have serious difficulty using inclines. Most ambulatory people and most people who use wheelchairs can manage
a slope of 1:16. Many people cannot manage a slope of 1:12 for 30 ft (9 m).
It also discusses the use of landings to break up the ramp, use of handrails and edge guards and the allowable cross slope.
Double check with your city, county or state to make sure that they don't have more stringent requirements.