Run this code to see what I mean: Go Playground demo of issue

package main

import (
    "fmt"
    "strconv"
)

func main() {
    // From https://golang.org/src/math/const.go
    var SmallestNonzeroFloat64AsString string = "4.940656458412465441765687928682213723651e-324"
    var SmallestNonzeroFloat64 float64
    var err error
    SmallestNonzeroFloat64, err = strconv.ParseFloat(SmallestNonzeroFloat64AsString, 64)
    if err != nil {
        panic(err)
    }
    fmt.Printf("SmallestNonzeroFloat64 = %g
", SmallestNonzeroFloat64)
    fmt.Printf("SmallestNonzeroFloat64 = %s
", strconv.FormatFloat(SmallestNonzeroFloat64, 'f', -1, 64))
}

SmallestNonzeroFloat64 is defined in math/const.go and I assumed it can be represented by a float64 variable.

But when it is parsed into a float64 with strconv.ParseFloat() and printed with strconv.FormatFloat() the result is rounded.

Instead of 4.940656458412465441765687928682213723651e-324 I get 5e-324 (or its non-exponent equivalent, which you can see in the Go Playground results). The result is rounded.

Is there a way to get back the 4.940656458412465441765687928682213723651e-324?

Or is it a bug?

This is not a bug.

You could ask Go to print more digits.

fmt.Printf("SmallestNonzeroFloat64 = %.40g
", SmallestNonzeroFloat64)
// 4.940656458412465441765687928682213723651e-324

However, 5e-324 and 4.94…e-324 are in fact the same value, so Go is not wrong printing 5e-324. This value (2^-1074) is the smallest positive number representable by Float64 (also known as double in other languages). Larger numbers are all multiples of this, e.g. the next smallest number would be 2 × 2^-1074 = 1e-323, the next would be 3 × 10^-1074 = 1.5e-323, etc.

In the other words, all numbers more precise than 5e-324 would not be representable in Float64. So it makes no sense to print more digit after the "5". And 5e-324 is certainly more readable than 4.94…e-324.