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# Day 24: flex-shrink calculation

This is another optional knowledge. But if you're like me and is curious how the browser calculates flex-shrink. Join me in this rabbit hole 🐰

The math behind flex-shrink is a bit more complicated then flex-grow. You need to take into account of it's existing proportion and shrink it accordingly to the flex shrink amount. Hence, a few more calculation involved. Again, if this is throwing you off. Skip it. You don't need to know this to understand Flexbox. Luckily the browser takes care of it for you, how wonderful 😌

24 flex shrink calculation

# Calculation

Indeed the calculation is a bit more complicated. But no worries, let's break it down we go through it step by step, you got this 💪

Here's the HTML and CSS we're working with:

HTML

<div class="parent">
  <div class="green"></div>
  <div class="yellow"></div>
</div>

CSS

.parent {
  width: 800px;
}
.green {
  width: 300px;
  flex-shrink: 4;
}
.yellow {
  width: 600px;
  flex-shrink: 6;
}

# Step 1: Breaking down the variables

This is the formula:

new width = width - (shrink space x shrink ratio)

Let's extract the variables required in the formula to this handy table we can fill in as we go:

Variables
width need to calculate
shrink space need to calculate
shrink ratio need to calculate

# Step 2: Fill in what we know

From the CSS value, we can conclude the following:

  • The parent element (container) has a width of 800
  • Green child element has a width 300 and flex-shrink of 4
  • Yellow child element has a width 600 and flex-shrink of 6

Let's update our chart with this information:

Green Yellow
flex shrink 4 6
width 300 600

# Step 3: Calculate "shrunk space"

This is the formula:

shrunk space = total children widths - parent width

Remember what we know:

  • The parent element (container) has a width of 800
  • The child elements has a width of 300, 600

Great, we can use that information to calculate "total children widths":

total children widths = green + yellow
                      = 300   + 600

=> 900

Now we can calculate our "shrunk space":

shrunk space = total children widths - parent width
             = 900                   -  800

=> 100

Let's update our chart and add the additional information:

Green Yellow Total
flex shrink 4 6
width 300 600
shrunk space - - 100

# Step 4: Calculate "shrink ratio"

This is the formula:

shrink ratio = (width x flex shrink) / total shrink scaled width

Notice this new variable, total shrink scaled width. So we need to calculate that first to get our shrink ratio.

# Step 4-1: Calculate "total shrink scaled width"

This is the formula:

total shrink scaled width = Σ(width x flex shrink)

"Σ" Sigma is a math symbol that means the summation of something. So we need to apply width x flex shrink for all the child elements.

Green

width x flex shrink = 300 x 4

=> 1200

Yellow

width x flex shrink = 600 x 6

=> 3600

Finally

total shrink scaled width = 1200 + 3600

=> 4800

Let's add this information to our chart:

Green Yellow Total
flex shrink 4 6
width 300 600
shrunk space - - 100
total shrink scaled width - - 4800

# Step 4-2: Back to calculating "shrink ratio"

Fantastic, now that we know the "total shrink scaled width", we can return with calculating the "shrink ratio". Remember the formula:

shrink ratio = (width x flex shrink) / total shrink scaled width

Green

shrink ratio = (300 x 4) / 4800

=> 0.25

Yellow

shrink ratio = (600 x 6) / 4800

=> 0.75

Let's add this information to our chart:

Green Yellow Total
flex shrink 4 6
width 300 600
shrunk space - - 100
shrink ratio 0.25 0.75

# Final step: Calculate "new width"

Remember the formula:

new width = width - (shrink space x shrink ratio)

Green

new width = 300 - (100 x 0.25)

=> 275

Yellow

new width = 600 - (100 x 0.75)

=> 525

Done! We have successfully calculated the new width 🥳

Green Yellow
width 300 600
shrunk space 4 6
shrink ratio 0.25 0.75
new width 275 525

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