The problem is that you try to parse "x.x\n", e.g: 1.8\n. And this returns an error: strconv.ParseFloat: parsing "1.8\n": invalid syntax. You can do a strings.TrimSpace function or to convert feet[:len(feet)-1] to delete \n character

With strings.TrimSpace() (you need to import strings package):

feetFloat, _ := strconv.ParseFloat(strings.TrimSpace(feet), 64)

Wtih feet[:len(feet)-1]:

feetFloat, _ := strconv.ParseFloat(feet[:len(feet)-1], 64)

Output in both cases:

10.8 feet converted to meters give you 3.2918400000000005 meters

Use strconv.FormatFloat like such:

s := strconv.FormatFloat(3.1415, 'f', -1, 64)
fmt.Println(s)

Outputs

3.1415

You need to convert string to float64 using strconv.ParseFloat function:

package main

import (
    "fmt"
    "os"
    "strconv"
)

var numbers []float64
var sum float64 = 0

func main() {

    if len(os.Args) <= 1 {
        return
    }

    for _, arg := range os.Args[1:] {
        if n, err := strconv.ParseFloat(arg, 64); err == nil {
            numbers = append(numbers, n)
        }
    }

    fmt.Println("Numbers are: ", numbers)
    for _, value := range numbers {
        sum += value
    }

}