There's a lot to consider when you're calculating takeoff performance. And if you don't understand what will affect your ability to takeoff, you might end up making a mistake like this...

1) Weight

Normally, lift-off speed is about 15% above stall speed. So if your weight increases, you'll have a higher stall speed and your lift-off speed will increase too. As a result, more time is needed to get to that lift-off speed, since more weight means slower acceleration, and you will use more runway.

2) Center of Gravity

With less tail-down force required for flight with an aft center of gravity, less lift needs to be produced by your wings, so your stall speed is lower and you can rotate sooner. But be careful not to over-rotate with an aft CG and pitch up into a stall.

3) Density Altitude

If an airport whose elevation is 500 MSL has a reported density altitude of 5,000 feet, aircraft operating to and from that airport will perform as if the airport elevation were at 5,000 feet MSL. So if it's hot outside, high in elevation, or humid, it'll take you longer to take off.

4) Wind

Windspeed and direction has a huge impact on takeoff performance. In the C172S POH, for every 9 knots of headwind, takeoff roll is reduced by 10%. And for tailwinds up to 10 knots, for every 2 knots of wind, takeoff roll is increased by 10% - That's 50% more distance you'll need to take off with a 10 knot tailwind!

5) Runway Condition

A dry, paved runway is what's used in calculating takeoff performance in your POH, but we're not always lucky to have perfect conditions. Grass runways increase takeoff and landing distance. Gravel and wet runways on the other don't increase takeoff distance, but increase your landing distance.

6) Runway Slope

Looking in the AF/D is important if you need to calculate takeoff distance. As a rule of thumb, an upslope of 1% causes a 22% increase in ground roll at sea level, while a 1% down slope only decreases the ground roll by 7%.

7) Pilot Technique

It all comes down to you, the PIC, in the end. Distances found in your pilot's operating handbook are calculated with techniques consistent with maximum performance pilot controls. For instance, if you choose to rotate at 55 knots vs the 48 knots listed in your POH, your overall takeoff distance could increase dramatically.

Easy enough, right? Knowing what affects your takeoff performance is pretty important if you want to fly into those fun, small airports.