H1-H6 font sizes in HTML

Question

In HTML (and in typography in general, I suppose), there appears to be some defined sizes for H1-H6 -elements.

Ie., if the baseline font size is 16px (or 100%), then h1 (w/c)ould be 2.25em (36px). And H2 (w/c)ould be 1.5em (24px). Et cetera. Where do these variables come from? The H1=36px, H2=24px, H3=21px, H4=18px, H5=16px, H6=14px, that is. (or, if you like, H1=2em, H2=1.5em, H3=1.17em, etc., the point isn't the numbers themselves, but the relation between them)

Is there some mathematical formula for them? Does it have something to do with golden ratio or fibonacci? I haven't been able to find information on this.

EDIT: to be more specific, what is the pattern; why does it go from 36 to 24 to 21, or start from 36 to begin with (or, if you like, from 2em to 1.5em to 1.17em, etc.)?

Oh, I forgot to specify where I came up with the original numbers, they're from here

Answer 1

The classic typographical scale is based on getting from H6 at the root font size up to H1 at double the size. The :root or html element is set to 100% and that usually translates to 16px. Anything set to 1.0rem takes on the value of the :root element, abreviated as rem.

To calculate the progression in size from 1.0rem to 2.0rem in 5 steps, namely H2, H3, H4, H5, H6 the mathematical formula is as shown:

where fi is the calculated value (repeated 5 times),
f0 is the initial value, 1.0rem in this case,
r is the ratio, 2 in this case since we want to go from 1.0rem to 2.0rem from H6 to H1,
i is the iteration we are on, so 1~5, and
n is the number of steps to get from H6 to H1, 5 in this case.

The result you get is as follows:

h1 { font-size: 2.00rem; }
h2 { font-size: 1.74rem; }
h3 { font-size: 1.52rem; }
h4 { font-size: 1.32rem; }
h5 { font-size: 1.15rem; }
h6 { font-size: 1.00rem; }

and it looks like this:

(See this article for more details)

Some designs are based on the golden ratio (r = 1.618034)

They set the progression higher and in only 2 steps:

h1 { font-size: 2.06rem; }
h2 { font-size: 1.62rem; }
h3 { font-size: 1.27rem; }

Others use em instead of rem in an effort to allow the end user's settings to control the progression as successive headings reference parent header sizes instead of always referencing the :root element size.

I personally use the typographic scale and rem units as I find I have a bit more control over the display and layout, yet allow the end user to set their :root value by setting mine to 100%. I also set the big, p and small sizes accordingly. I therefore use:

  big { font-size: 2.29rem; }
    p { font-size: 1.00rem; }
small { font-size: 0.87rem; }

Answer 2

Late to the party, but here's a fun mixin for Sass/SCSS that does some fibonacci math for you based on a starting font-size value for the H1 tags:

@use "sass:map";
@mixin headers($h1-em: 2em) {
    $fib: ("6": .236, "5": .382, "4": .50, "3": .618, "2": .786, "1":1);
    @for $i from 1 through 6 {
        h#{$i} {
            font-size: $h1-em * map.get($fib, #{$i})};
        }
      
}

Answer 3

Update: Nowadays, modern browsers use these values.

Original answer:

They are defined by each browser maker independently.

They are not uniform across browsers and are there for semantics (Large header, slightly smaller header etc...).

If you look at the HTML 4 specification for these, there no mention of how they are supposed to be styled, only that they should be. From the spec:

Visual browsers usually render more important headings in larger fonts than less important ones.

If you want these to be consistent, you should use a reset stylesheet that defines them.

Even though w3 has defined a suggested default stylesheet for HTML 4 with the following details, most browsers ignore this suggestion:

h1              { font-size: 2em; margin: .67em 0 }
h2              { font-size: 1.5em; margin: .75em 0 }
h3              { font-size: 1.17em; margin: .83em 0 }
h5              { font-size: .83em; margin: 1.5em 0 }
h6              { font-size: .75em; margin: 1.67em 0 }
h1, h2, h3, h4,
h5, h6          { font-weight: bolder }

(yes, I see no font-size: for h4)

Answer 4

I know this post is old. I came across it with the same question, where do they get this from. I think I found it.

It is a derivation of a pentatonic music scale. The Type scale is anyway. The Headings are taken from the Type scale, though not in a 1:1 order.

The scale goes: 8 9 10 12 14 16 18 21 24... The scale doubles in 5 steps (12 to 24). Each step is the base(12) times 2(the scale['it doubles']) to the power of i(step) divided by 5(ttl steps)['i/5'] - rounded to the nearest.

So h4 is the base, h3 is step 1, h2 is step 3, and h1 is step 5, or the octive of 12 on a pentatonic scale. h5 and h6 are 1 and 3 steps from base the other way.If I understand this, it would be the equivalent of a, E major chord.

That took me about 2 hours to figure out with a spreadsheet and an explanation of musical scales.

Answer 5

Many of them say different sizes for heading tags but there was a variation from bootstrap to default font-size.here mentioned are default font-sizes:

h1 { font-size: 24px;}
h2 { font-size: 22px;}
h3 { font-size: 18px;}
h4 { font-size: 16px;} 
h5 { font-size: 12px;} 
h6 { font-size: 10px;}

Answer 6

For bootstrap the variation of heading tags font-sizes in pixels are as follows check this bootstrap headings

h1 - 36px)
h2 - 30px
h3 - 24px
h4 - 18px
h5 - 14px
h6 - 12px

Answer 7

One possible progression approach is to use square roots, via a formula such as 2/sqrt[heading#]. Hence, you'd have:

H1 = 2/sqrt1 = 2
H2 = 2/sqrt2 = 1.414
H3 = 2/sqrt3 = 1.155
H4 = 2/sqrt4 = 1
H5 = 2/sqrt5 = 0.894
H6 = 2/sqrt6 = 0.816

For a font base of 12, that'd be close enough to 24, 17, 14, 12, 11, 10. For other bases, the results may be a bit farther away from integers.

Fibonacci would work well with base 16, so you'd have:

H1=32
H2=24
H3=19
H4=16
H5=14
H6=13

Answer 8

It is browser-dependant, as other say.

On the other side, there is a rule in typography to set font sizes: if the base font has size X, the larger fonts should grow exponentially; the usual way is to have sizes X*sqrt(2), X*sqrt(2)^2, X*sqrt(2)^3 and so on, but you can change the base.

However, computer fonts have some special requirements.

They used to be provided in a bitmap form only (so the sizes were fixed), and even when provided in vector form -- some formats preferred some special sizes: divisible by 2 or 5 (this was f.e. the case with Amiga's old vector fonts... Agfa Intellifont?).

Even now font engines like integer sizes more, because their hinting algorithms work better.

And people seem to got used to the values chosen because of these technical restrictions, even though font engines got much better now.

Answer 9

Some nominal figures:

Default style sheet for HTML 4:

• h1: 2em
• h2: 1.5em
• h3: 1.17em
• h4: 1em
• h5: 0.83em
• h6: 0.75em

Firefox 3 and Safari 4 (actually, WebCore):

• h1: 2em
• h2: 1.5em
• h3: 1.17em
• h4: 1em
• h5: 0.83em
• h6: 0.67em

Answer 10

In a more general way, related sizes like this are often based on a geometric series, i.e. each successive number is bigger by a constant factor (something like 1.2, or sqrt(2)) from the previous one. There is the same kind of progression in the size of wrenches and hexagonal keys, or screw diameters, etc in mechanics, or in the A5/A4/A3… family of paper sizes.

Answer 11

W3C suggested a default rendering stylesheet for browsers to implement.

You'll notice that your figures differ from them. That's because browser makers have a habit of ignoring everything W3C say.

Answer 12

I think it is up to the browser, which may even even let the user define the font sizes (I remember opera doing that). The HTML spec doesn't go into much detail:

There are six levels of headings in HTML with H1 as the most important and H6 as the least. Visual browsers usually render more important headings in larger fonts than less important ones.

This is intentional since HTML is designed to describe the structure, not the presentation of the document.