Exact size depends on many factors. I'll just show you how to calculate different metrics of font.

font = ImageFont.truetype('arial.ttf', font_size)
ascent, descent = font.getmetrics()
(width, baseline), (offset_x, offset_y) = font.font.getsize(text)

• Height of red area: offset_y
• Height of green area: ascent - offset_y
• Height of blue area: descent
• Black rectangle: font.getmask(text).getbbox()

Hope it helps.

The top voted answer is outdated. There is a new function in Pillow 8.0.0: ImageDraw.textbbox. See the release notes for other text-related functions added in Pillow 8.0.0.

Note that ImageDraw.textsize, ImageFont.getsize and ImageFont.getoffset are broken, and should not be used for new code. These have been effectively replaced by the new functions with a cleaner API. See the documentaion for details.

To get a tight bounding box for a whole string you can use the following code:

from PIL import Image, ImageDraw, ImageFont
image = Image.new("RGB", (200, 80))
draw = ImageDraw.Draw(image)
font = ImageFont.truetype("arial.ttf", 30)

draw.text((20, 20), "Hello World", font=font)
bbox = draw.textbbox((20, 20), "Hello World", font=font)
draw.rectangle(bbox, outline="red")
print(bbox)
# (20, 26, 175, 48)

image.show()

You can combine it with the new ImageDraw.textlength to get individual bounding boxes per letter:

from PIL import Image, ImageDraw, ImageFont
image = Image.new("RGB", (200, 80))
draw = ImageDraw.Draw(image)
font = ImageFont.truetype("arial.ttf", 30)

xy = (20, 20)
text = "Example"
draw.text(xy, text, font=font)

x, y = xy
for c in text:
  bbox = draw.textbbox((x, y), c, font=font)
  draw.rectangle(bbox, outline="red")
  x += draw.textlength(c, font=font)

image.show()

Note that this ignores the effect of kerning. Kering is currently broken with basic text layout, but could introduce a slight inaccuracy with Raqm layout. To fix it you would add the text length of pairs of letters instead:

for a, b in zip(text, text[1:] + " "):
  bbox = draw.textbbox((x, y), a, font=font)
  draw.rectangle(bbox, outline="red")
  x += draw.textlength(a + b, font=font) - draw.textlength(b, font=font)

from PIL import Image, ImageDraw, ImageFont

im = Image.new('RGB', (400, 300), (200, 200, 200))
text = 'AQj'
font = ImageFont.truetype('arial.ttf', size=220)
ascent, descent = font.getmetrics()
(width, height), (offset_x, offset_y) = font.font.getsize(text)
draw = ImageDraw.Draw(im)
draw.rectangle([(0, 0), (width, offset_y)], fill=(237, 127, 130))  # Red
draw.rectangle([(0, offset_y), (width, ascent)], fill=(202, 229, 134))  # Green
draw.rectangle([(0, ascent), (width, ascent + descent)], fill=(134, 190, 229))  # Blue
draw.rectangle(font.getmask(text).getbbox(), outline=(0, 0, 0))  # Black
draw.text((0, 0), text, font=font, fill=(0, 0, 0))
im.save('result.jpg')

print(width, height)
print(offset_x, offset_y)
print('Red height', offset_y)
print('Green height', ascent - offset_y)
print('Blue height', descent)
print('Black', font.getmask(text).getbbox())

result

Calculate area pixel

from PIL import Image, ImageDraw, ImageFont

im = Image.new('RGB', (400, 300), (200, 200, 200))
text = 'AQj'
font = ImageFont.truetype('arial.ttf', size=220)
ascent, descent = font.getmetrics()
(width, height), (offset_x, offset_y) = font.font.getsize(text)
draw = ImageDraw.Draw(im)
draw.rectangle([(0, offset_y), (font.getmask(text).getbbox()[2], ascent + descent)], fill=(202, 229, 134))
draw.text((0, 0), text, font=font, fill=(0, 0, 0))
im.save('result.jpg')
print('Font pixel', (ascent + descent - offset_y) * (font.getmask(text).getbbox()[2]))

result