I’ve been using customtkinter for a little while now, and I have a situation where I want to find the exact, pixel-perfect width of a CTkLabel from the font size and length of the displayed string. However, none of the methods I’ve tried are accurate. Here’s a setup that demonstrates the issue:

import customtkinter as ctk


class MeasuringLabel(ctk.CTkLabel):
    def __init__(self, master, font_size=30, length=20, **kwargs):
        self.font_size = font_size
        self.length = length
        self.font = ctk.CTkFont('Courier new', size=self.font_size)
        self.text = '0' * self.length # Sample text
        super().__init__(master=master, font=self.font, text=self.text,
                         fg_color='red', **kwargs)

    # Measure using ctk.CTkFont.measure
    def measured_width(self):
        return self.font.measure(self.text, displayof=self)

    # Measure one character and extrapolate
    def extrapolated_width(self):
        return self.font.measure('0', self) * self.length

    # Take 0.6*font_size as the width of one character and extrapolate, then round
    def theoretical_width(self):
        return round(self.font_size * MeasuringLabel.font_width_to_size * self.length)

    # Same as above, but round the character width first
    def theoretical_round_first_width(self):
        return round(self.font_size * MeasuringLabel.font_width_to_size) * self.length

    # Get the width of the label using the specified method
    def get_width(self, width_method='measured'):
        width_function = MeasuringLabel.width_functions[width_method]
        return width_function(self)

    # Return a CTkFrame with the width as measured using the specified method
    def width_bar(self, width_method='measured'):
        width = self.get_width(width_method)
        print(f'{width_method+": ":<25} {width}')
        return ctk.CTkFrame(self.master, width=width, height=10,
                            corner_radius=0, fg_color='blue')


    # This is specific to Courier new. The exact value is 1229/2048 according to
    # /a/19114963/.
    font_width_to_size = 0.6

    # These functions, when called with a MeasuringLabel instance as the argument,
    # return the width as calculated using that method.
    width_functions = {
        'measured': measured_width,
        'extrapolated': extrapolated_width,
        'theoretical': theoretical_width,
        'theoretical_round_first': theoretical_round_first_width,
        'winfo_width': ctk.CTkLabel.winfo_width,
        'winfo_reqwidth': ctk.CTkLabel.winfo_reqwidth,
    }


class App(ctk.CTk):
    def __init__(self, font_size, length):
        super().__init__()

        self.title('Width measuring')
        self.geometry('600x200')

        self.label = MeasuringLabel(self, font_size, length)
        self.label.grid(row=0, column=0, sticky='w') # Left-align
        
        self.update_idletasks() # Update, so winfo_width returns something useful

        for i, width_method in enumerate(MeasuringLabel.width_functions):
            new_bar = self.label.width_bar(width_method)
            new_bar.grid(row=i+1, column=0, pady=(2,0), sticky='w') # Left-align


app = App(font_size=30, length=20)
app.mainloop()

Essentially, I have 6 different methods in the MeasuringLabel class (2 of which are inherited from CTkLabel) for measuring the width. When I initialize the app, it creates a MeasuringLabel with the specified font_size and with text = '0' * length. It then creates 6 CTkFrames whose widths come from the different width methods, in the hopes that one will have the same width as the MeasuringLabel, regardless of font_size and length. However, none of them are accurate:

font_size=30, length=20:

font_size=37, length=20:

What is immediately clear is that winfo_width and winfo_reqwidth are always too long, and therefore useless. I suspect this might have something to do with screen scaling, so if someone knows how to compensate for this, I’d be very grateful.

Moreover, the first, second, and fourth bars are always equal. This leads me to believe that the CTkFont.measure method calculates the widths of the characters individually, rounding to ints, and adds them up. I don’t understand why this wouldn’t give the same width as the MeasuringLabel though.

In a final effort, I tried to manually set the width of the bar for each font size. I got the following for length=20:

# The keys are the font_sizes, and the values are the correct widths.
empirical_widths = 
    {15: 176, 16: 192, 17: 208, 18: 208, 19: 224,
     20: 240, 21: 256, 22: 272, 23: 272, 24: 288, 
     25: 304, 26: 304, 27: 320, 28: 336, 29: 352,
     30: 368, 31: 368, 32: 384, 33: 400, 34: 400,
     35: 416}

The worst part is that it isn’t even linear. I made this graph to compare the different methods:

Closeup around font_size=24:

Again, measure, extrapolated, and theoretical_round_first overlap (red), and so do winfo_width and winfo_reqwidth (gray). And none of them are correct (black) more than occasionally.