Buoyancy Calculator: How to Figure Out How Much Weight You Need for Scuba Diving | Sport Diver

Buoyancy Calculator: How to Figure Out How Much Weight You Need for Scuba Diving

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How much weight do you need? It varies by diver.

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How much weight do you need to wear as a diver? Correct weighting depends on your personal buoyancy needs and is influenced by a number of factors — from the composition of your body to the thickness of your wetsuit. You can get a rough estimate of how much weight you need by using our buoyancy calculator.

The rule you often hear — that you should float at eye level — is about right for the end of your dive, when you will weigh 5 pounds less. (Most of that loss is the weight of the breathing gas you will use up.) So you need about 5 pounds of extra lead at the beginning of the dive.

Divers are generally taught to define this as being neutrally buoyant at 15 feet deep while wearing an empty BC and carrying a nearly empty tank. But how do you get there? There’s the basic ballpark method — carry 10 percent of your body weight in lead. Or there’s the surface float method — in full scuba gear, load enough weight to enable you to float with the water at eye level (some would say at the hairline).

Here’s how:

STEP 1: Calculate for Your Body

How much weight do you need to make your body neutral? Take a few weights into the water wearing just a swimsuit. You will be perfectly weighted when you can hang motionless with half a breath, and sink when you exhale. (Using a snorkel can make this test easier.)

Tip for Shaving Ballast Weight: Lose weight. Also, work to turn build some muscle. Fat mass is a lot more buoyant than muscle mass, so any fat you can lose and any muscle you gain will lower your buoyancy deficit. You'll reap plenty of health benefits — and be more fit for diving, too.


READ MORE: Tips for Staying Warm while Scuba Diving


STEP 2: Calculate for Your Exposure Suit

Wearing your exposure suit, get into the water and repeat the procedure outlined in Step 1. Then take the total amount of weight required to get neutral, subtract Step 1’s total, and you’ll have the net buoyancy budget for your exposure suit.

Tip for Shaving Ballast Weight: If water conditions permit, cut down on the thickness of your wetsuit. A wetsuit can have two to three pounds of buoyancy for every millimeter of thickness. If you wear a neoprene drysuit, consider that compressed or crushed neoprene suits have much less buoyancy than standard neoprene. If you wear a fabric drysuit, remember that thinner undergarments have much less buoyancy than the puffy stuff.

STEP 3: Calculate for Your BC

BCs can be a huge source of inherent buoyancy, especially the older, full-featured models that have lots of traditional-style padding. It used to be common for BCs to carry upwards of four pounds-plus of inherent buoyancy, which means, of course, that you need four pounds-plus of extra lead on your weight belt to compensate for it. Fortunately, most modern BCs carry much less inherent buoyancy.

To test your BC’s inherent buoyancy, submerge it while venting all exhaust valves to bleed air from the bladder. Knead the padding in the shoulders and backpad and behind the pockets to release air bubbles. Slowly rotate the BC to enable any trapped air to escape. Be patient, allow plenty of time for water to displace the air in the material. When you stop seeing bubbles, release the BC into the water column. If it heads to the surface you’ve got some inherent buoyancy to deal with. Add weights until the BC will hang neutrally buoyant in the water. Then count up how many weights it took to get there and you’ll have your number.

Tip for Shaving Ballast Weight: Buy a modern BC. Models that have come onto the market within the last three or four years carry, on average, from one to 2.5 pounds of inherent buoyancy, and some carry none at all. Note: while most manufacturers don’t provide the inherent buoyancy of their BCs, you can always find that info in ScubaLab BC reviews on our sister site, scubadiving.com


READ MORE: Scuba Diving Hand Signals Every Diver Should Know


STEP 4: Calculate for Your Tank

The buoyancy characteristics of tanks vary widely. For example, a standard aluminum 80 is 1.6 pounds negatively buoyant when topped off, and 2.8 pounds positively buoyant at 500 psi. That’s close to a four and a half pound buoyancy differential between the beginning of a dive and the end of a dive that, of course, needs to be dealt with by adding ballast weight.

A steel tank, on the other hand, tends to start off negatively buoyant and stay that way. For example, a high-pressure 80 is about nine pounds negative when full and three pounds negative when empty. That’s three pounds that can be removed from your weight system.

Tip for Shaving Ballast Weight: Switch from an aluminum cylinder to a steel cylinder. A properly weighted diver who goes from an aluminum 80 to, say, a HP steel 80 could theoretically take six pounds off his weight belt.

STEP 5: Calculate for Everything Else

Gather your reg, gauges, knife, fins and any other items you regularly dive with, place them in a neutrally buoyancy mesh bag, and submerge it. The goal here is primarily to see if the total package is positively buoyant. If it is, add some weight until it becomes neutral. If it’s negative it probably won’t be by much, so consider it a ballast slush fund. It’s not working against you, and that’s all that matters.

STEP 6: Put It All Together

Add it all up. This should be very close to your target ballast weight requirements, and it should also give you a clear picture of where your biggest buoyancy challenges lie. To double-check your calculations, gear up with all the components you measured separately, get back into the water and repeat Step 1. If the above scenario played out like it’s supposed to, you should be floating at eye or forehead level in a relaxed position. When you exhale you should start to slowly sink. If not, you couldn’t be more than a pound or so off your target. Make the final adjustment and go diving.

ADJUSTING WEIGHT FOR SALTWATER AND FRESHWATER

By how much do you have to change your weight belt when going from fresh to salt water? To be accurate, you have to consider the whole package: diver plus equipment.

Stand on the bathroom scale with the equipment and weight that makes you neutral in one medium or the other. Or you can estimate. A standard aluminum 80 tank weighs 32 pounds, a 7mm wetsuit is about 8 pounds. For regulator, mask, fins, etc., figure about 15 pounds. Include your weights.

Going from fresh water to salt? Multiply by 0.025 and add that amount.

Going from salt water to fresh water? Multiply the total by 0.025 and subtract that amount from your weight belt or integrated-weight BC.

Example: You weigh 175 pounds and your equipment adjusted for salt water weighs 75 pounds, for a total of 250 pounds. 250 x 0.025 = 6.25. Subtract 6 pounds from your weight belt for fresh water.

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