How to convert DP, PX, SP among each other, especially DP and SP?

DP to PX:

public static int dpToPx(float dp, Context context) {
    return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, context.getResources().getDisplayMetrics());
}

SP to PX:

public static int spToPx(float sp, Context context) {
    return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, sp, context.getResources().getDisplayMetrics());
}

DP to SP:

public static int dpToSp(float dp, Context context) {
    return (int) (dpToPx(dp, context) / context.getResources().getDisplayMetrics().scaledDensity);
}

The accepted answer is missing a few useful conversions.

SP to PX

float sp = 20;
float px = sp * getResources().getDisplayMetrics().scaledDensity;

or

float px = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, sp, getResources().getDisplayMetrics());

PX to SP

float px = 70;
float sp = px / getResources().getDisplayMetrics().scaledDensity;

DP to PX

float dp = 20;
float px = dp * getResources().getDisplayMetrics().density;

or

float px = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, getResources().getDisplayMetrics());

PX to DP

float px = 70;
float dp = px / getResources().getDisplayMetrics().density;

Notes

• The floats I chose above (20 and 70) were arbitrary values. You can plug in different numbers if you like.
• px refers to pixels. The number of pixels that a device has per inch of screen space is called the density.
• dp means density-independent pixels. That is, no matter what device is used, the actual size should be the same. For example, if I set a view to be 100 dp wide, it will have the same width on a new high density phone as it does on an old low density phone. (If I had set the width to 100 px, on the other hand, it would appear large on a low density phone and small on a high density phone.) Density is measured in dots per inch (DPI). The formula is px = dp * density. So you just multiply or divide by the density to convert between px and dp.
• sp means scale-independant pixels. It is just used for fonts, not views. It is similar to dp except it also factors in the user preferences. This density with user preferences taken into account is known as scaled density. Setting a TextView font to a size of 30 sp, for example, will make the text generally appear to be the same physical size on all devices. However, your grandmother may have her preferred font size maxed all the way up in her phone settings, so 30 sp text will look bigger on her phone than it does on yours. The formula is px = sp * scaledDensity.
• Meaning of DP and SP
• DP to SP conversion is not generally useful