how to measure an object?

edit

I'm trying to make a tool that does object measurement when it is pointed at the webcam, but it ends up measuring anything.

code:

import cv2
import numpy as np

cap = cv2.VideoCapture(0)

while True:
    _, frame = cap.read()

    frame = frame[0:350, 15:540]

    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    gray = cv2.GaussianBlur(gray, (7, 7), 3)

    _, threshold = cv2.threshold(gray, 100, 255, cv2.THRESH_BINARY|cv2.THRESH_TRIANGLE)

    kernel = np.ones((5,5),np.uint8)

    dilation = cv2.erode(threshold,kernel,iterations = 1)

    contours,_ = cv2.findContours(dilation, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)


    for (i,c) in enumerate(contours):
        (x,y,w,h) = cv2.boundingRect(c)
        area = cv2.contourArea(c)

        cv2.rectangle(frame,(x,y),(x+w,y+h),(0,255,0),2, cv2.LINE_AA)
        cmY = (h*15)/120
        cmX = (w*10)/71
        cv2.putText(frame, str(int(cmY)), (x, y+h+15), cv2.FONT_HERSHEY_SIMPLEX, 0.4, (0, 255, 255),1)
        cv2.putText(frame, str(int(cmX)), (x+w+15, y+h+15), cv2.FONT_HERSHEY_SIMPLEX, 0.4, (0, 255, 255),1)


    cv2.imshow("Frame", frame)


    key = cv2.waitKey(1)
    if key == 27:
        break

cap.release()
cv2.destroyAllWindows()