How to find max value for Double and Float in Swift

As of Swift 3+, you should use:

CGFloat.greatestFiniteMagnitude
Double.greatestFiniteMagnitude
Float.greatestFiniteMagnitude

While there’s no Double.max, it is defined in the C float.h header, which you can access in Swift via import Darwin.

import Darwin

let fmax = FLT_MAX
let dmax = DBL_MAX

These are roughly 3.4 * 10^38 and 1.79 * 10^308 respectively.

But bear in mind it’s not so simple with floating point numbers (it’s never simple with floating point numbers). When holding numbers this large, you lose precision in a similar way to losing precision with very small numbers, so:

let d = DBL_MAX
let e = d - 1.0
let diff = d - e
diff == 0.0  // true

let maxPlusOne = DBL_MAX + 1
maxPlusOne == d  // true

let inf = DBL_MAX * 2
// perhaps infinity is the “maximum” 
inf == Double.infinity  // true

So before you get into some calculations that might possibly brush up against these limits, you should probably read up on floating point. Here and here are probably a good start.

AV's answer is fine, but I find those macros hard to remember and a bit non-obvious, so eventually I made Double.MIN and friends work:

extension Double {
    static var MIN     = -DBL_MAX
    static var MAX_NEG = -DBL_MIN
    static var MIN_POS =  DBL_MIN
    static var MAX     =  DBL_MAX
}

Don't use lowercase min and max -- those symbols are used in Swift 3.