Android

test.kt

val paint = TextPaint().also {
    it.textSize = 10
    if (!isInEditMode) {
        it.typeface = ResourcesCompat.getFont(context, R.font.your_font)
    }
}
paint.measureText("hoge")

iOS

UIFont+.swift

import UIKit

extension UIFont {
    /// テキストのwidthを取得する
    func measureText(text: String) -> CGFloat {
        let str = NSString(string: text)
        return str.size(withAttributes: [.font: self]).width
    }
}

test.swift

let font = UIFont(name:"your_font", size: 10.0)
font.measureText("hoge")