The size of an anchor correlates to the size of the boat and the conditions where the anchor will be used. A Danforth anchor is measured with holding power, not weight or size. Anchors are tested and rated by holding power to match the size of the boat and conditions. Also, consider the length and weight of the anchor chain and rope to match the boat size and weight.  Mushroom anchors are rated by weight. A smaller boat under 16 feet long can be secured with a 12-pound anchor. Larger boats require a heavier anchor. For docking or heavy conditions, add a length of heavy chain equal to the length of the boat. Grapnel anchors are best for kayaks and canoes. Match the size of the anchor to the water depth and conditions. In shallow, calm water, a 1.5-pound anchor will work. In deeper water or swift current, use a 7-pound anchor. Use the lightest anchor required to reduce weight in the kayak or canoe.