Markup is profit measured against what a job cost you. Margin is profit measured against what you charged for it. They sound close enough to use interchangeably — which is exactly why so many quotes are underpriced by contractors who did the math correctly, just with the wrong one of the two.
Worked example
A job costs $1,000. Aim for a 30% markup, and the price is $1,300 — cost plus 30% of cost. Run the numbers on that $1,300 price, though, and the actual margin is only 23%, not 30%. Aim for a 30% margin instead, and the price needs to be $1,000 ÷ (1 − 0.30) = $1,428.57. That's a $128 gap on a single mid-size job, multiplied across every job you price the same way.
Which one should you be targeting
Margin is the more useful number to run a business on, because it's what actually shows up on your P&L as profit. "I want a 30% margin" is a statement about how much of every dollar collected is profit. "I want a 30% markup" is a statement about cost, one step removed from the number that matters.
The fix that takes thirty seconds
Decide which one you're quoting on, label it explicitly in whatever tool or spreadsheet you use, and stop eyeballing the conversion. A margin calculator that does the division for you removes the single most common arithmetic mistake in service-business pricing.