General:
* Calculate: To determine the amount or number of something mathematically.
* Calendar: A system of organizing time, usually a table showing days, weeks, and months.
* Calamity: A serious event causing great loss or suffering.
* Calibrate: To adjust or standardize an instrument or measurement.
* Callous: Showing a lack of feeling or sympathy.
* Calm: Peaceful, tranquil, or undisturbed.
* Calorie: A unit of energy.
* Call: To shout or speak loudly to attract attention.
* Caliber: The diameter of a bullet or the quality of something.
Less Common:
* Calcine: To heat something to a high temperature to remove impurities.
* Calico: A plain-woven cotton cloth, often printed.
* Calisthenics: Exercises done without equipment.
* Callow: Inexperienced and immature.
* Calomel: A type of mercury compound.
* Calvary: A place of execution or suffering.
* Calyx: The outer part of a flower that encloses the bud.
* Calx: A powdery residue left after heating a substance.
* Callus: A thickened area of skin.
Note: Some words may also be spelled with a "k" instead of a "c" before the "al" (e.g., "kalescence" instead of "calescence").