This tool will calculate the mass and number of parachutes that are necessary to slow down your landing craft to a given descent velocity, at a given elevation on the planet or moon (with an atmosphere!) of your choosing. Like Kerbal Space Program, this tool is still in development.
How to Use
First you must select which
planet or moon (with an atmosphere) you'd like to land on.
Next, input the
mass of your landing craft. You can find this value easily in KSP v.20.0 or newer by loading just your landing ship on the launch pad, exiting to the orbital map view (pressing m), and clicking on the info button on the far right of your screen. You may also use various plugins to find your ship's mass, or estimate the mass yourself.
Then, input the
drag coefficient of your spacecraft. The default value is 0.2 as the majority of parts in KSP have this coefficient. Small variations from this value will not significantly affect the calculations so using the default value will be sufficient most of the time.
After that, select the
desired landing velocity. Typically, 5 to 10 m/s is survivable for a sturdy ship, and 2 to 5 m/s for more delicate craft.
Finally, choose the
elevation of your landing site. On worlds with oceans, an elevation of 0 m is sea level. A table at the bottom of this page list the elevation ranges on different worlds, or if you know exactly where you'd like to land, you can estimate your landing elevation by visiting
Kerbal Space Program Maps, selecting the target planet/moon and choosing the 'Color Relief' layer.
When you're finished, click
Calculate to crunch the numbers, or
Reset to start over.
Total Parachute Mass Required
The calculator will tell you the total mass of parachutes that you need to add to your landing craft to slow down to your desired velocity. Simply attach any number or combination of nondrogue parachutes to your landing craft so that their total mass is close to required amount.
Example: If your landing craft has a mass of 10.0 tonnes with a drag coefficient of 0.2, and you wish to land on a site on Duna with an elevation of about 1000 m at a speed of 8 m/s, you will require about 1.5 tons of parachutes. You could use 10 radial Mk2R parachutes (0.15 tonnes each) to do the job, or 5 Mk16XLs (0.3 tonnes each), or a combination of 3 Mk16XLs and 4 Mk2Rs.
*Note: This value assumes a parachute drag coefficient of 500 and thus will not work with the stock drogue parachute;
Mk25 , but will work with the other stock parachutes;
Mk16,
Mk16XL or the
Mk2R. This value should theoretically work on custom/modded parachutes provided that they have a drag coefficient of 500.
Parachute Drag Required
This value tells you the total drag value from the parachutes required to slow your ship down to the desired velocity. This number is useful if you want to use parachutes with a drag coefficient other than 500, such as drogue chutes. The sum of the mass of the parachutes, multiplied by their respective drag coefficients should add up to this value.
Example: If your landing craft has a mass of 26.0 tonnes with a drag coefficient of 0.2, and you wish to land on a site on Eve with an elevation of 2000 m at a speed of 5 m/s, you will require a parachute drag of about 1001. You could use three
Mk25 drogue parachutes (0.2 tonnes, 170 drag each) and six
Mk16XL parachutes (0.3 tonnes, 500 drag each). The total drag of this combination is (3 * 0.2 * 170) + (6*0.3*500) = (102) + (900) = 1002.
Adding Parachutes by Number
You can now calculate the
number of stock parachutes required to slow you down to a given landing speed. Additionally, you can add or subtract any number of any type of stock parachute (including drogue chutes!) and see what effect that will have on your estimated landing speed.
When you click
Calculate the calculator will automatically tell you the number of MK16XL parachutes required to achieve your desired landing speed. You can choose to see the number of other parachutes required to slow you down to your desired speed by clicking the
ALL button next to it. You can then choose to increase or reduce the number of parachutes or mix and match any combination of parachutes using the
+ and
 buttons. Your
total parachute drag will update with every button click, but more importantly, so will your
estimated landing velocity!
Tips
The lower the elevation you choose, the less parachutes you will need to slow down to a given velocity. However, it will also become more difficult to lift off and return to orbit from a lower elevation than a higher one.
If it is impossible or impractical to slow down to your desired speed on parachutes alone, you might want to consider a powered descent in addition to (or instead of) using parachutes.
The following tables are accurate for KSP v 0.21.1 
World  MinElevation  MaxElevation 
Kerbin:  0m  6761m 
Eve:  0m  7540m 
Duna:  124m  8264m 
Laythe:  0m  6050m 

Parachute  Drag  Mass 
Mk16XL  500  0.3 tonnes 
Mk16  500  0.1 tonnes 
Mk2R  500  0.15 tonnes 
Mk25 (drogue)  170  0.2 tonnes 