Hydration deficit explained (with formulas, examples, and FAQs)

This page is intentionally thorough so your calculator can rank, get bookmarked, and earn backlinks. It answers the questions people actually search: “How dehydrated am I?”, “How much water did I lose?”, and “How much should I drink to recover?”

What is a hydration deficit?

A hydration deficit is the net amount of fluid your body is missing compared to baseline. It’s most obvious after sweating (exercise, heat, sauna), but it can also happen after long flights, illness, or simply forgetting to drink. People often describe it as “I feel dry, headachy, sluggish, and my performance fell off a cliff.”

The simplest way to estimate hydration deficit is to use body weight change across a short window. That works because in the short term, a sudden drop on the scale is mostly water. Fat loss doesn’t happen that fast, and muscle tissue doesn’t vanish in an hour. So when you step on a scale after a run and you’re down 0.8 kg, that’s very likely ~0.8 liters of body water—unless you also drank a lot during the session (in which case you lost even more, but replaced some of it).

The core formula (weight method)

This calculator uses a practical sports-hydration approach. In metric units, the base assumptions are:

• 1 kg of short-term body weight loss ≈ 1 liter of fluid loss.
• Fluids you drink during the session reduce net deficit, but do not reduce sweat loss.
• Bathroom breaks (urine) are fluid leaving the body, so they increase sweat-loss estimate.

We estimate total sweat loss (fluid lost during the session) like this:

Sweat loss (L) = (Pre-weight − Post-weight) (kg) + Fluid intake (L) − Urine (L)

Then we estimate your current net hydration deficit (how much you’re short right now) as:

Hydration deficit (L) = max(0, Sweat loss − Fluid intake)

Why show both? Because people confuse them. If you lost 1.2 L in sweat but drank 0.7 L during the workout, your sweat loss is 1.2 L (what left you), while your deficit is about 0.5 L (what you still need to replace).

Dehydration percentage

Many training plans and heat-safety guides talk about dehydration as a percentage of body weight. That’s calculated from the net deficit relative to pre-session weight:

Dehydration % = (Net deficit (L) ÷ Pre-weight (kg)) × 100

The calculator reports dehydration % after accounting for fluids you drank during the session. That makes it more representative of “how far from baseline you are right now.” It’s still an estimate—scales have noise, your clothing can hold sweat, and you can lose water through breathing—but it’s usually close enough to guide a smart recovery plan.

Example 1: simple post-run check

You step on the scale before a run at 75.0 kg. After the run, towel-dry, then weigh again at 74.2 kg. During the run you drank 0.3 L of water and didn’t use the bathroom.

• Weight loss = 75.0 − 74.2 = 0.8 kg
• Sweat loss = 0.8 + 0.3 − 0 = 1.1 L
• Hydration deficit = 1.1 − 0.3 = 0.8 L
• Dehydration % ≈ 0.8 ÷ 75.0 × 100 = 1.07%

In plain English: you likely lost ~1.1 liters during the run, replaced ~0.3 liters while running, and you’re still short about 0.8 liters afterward. Your dehydration level is ~1.1%, which is a “noticeable” range for many people—especially in heat.

Example 2: hot gym session with sipping

Pre = 180 lb, post = 178.6 lb, you drank 24 fl oz, and you estimate 6 fl oz urine during the session (yes, this is a guess).

First convert: 180 lb = 81.65 kg, 178.6 lb = 81.01 kg, 24 fl oz ≈ 0.71 L, 6 fl oz ≈ 0.18 L.

• Weight loss = 81.65 − 81.01 = 0.64 kg
• Sweat loss = 0.64 + 0.71 − 0.18 = 1.17 L
• Deficit = 1.17 − 0.71 = 0.46 L
• Dehydration % ≈ 0.46 ÷ 81.65 × 100 = 0.56%

Even though you lost a lot of fluid overall, the sipping helped keep your net deficit modest. That’s the point of drinking during a session: not “perfect hydration,” but preventing a large performance drop.

How the rehydration plan is built

A common practical rule is to drink about 100–150% of your measured deficit over the next few hours, because not all the fluid you drink is retained (you’ll lose some through urine, ongoing sweat, and respiration). This calculator uses your deficit as the base and then builds a simple schedule:

• Right now: a small starter dose so you don’t “chug and regret.”
• Next 60–120 minutes: the bulk of rehydration, split into manageable amounts.
• Later: top-off and optional electrolytes, especially if you trained hard or sweat heavily.

If you select a higher heat/intensity level, the calculator adds a small buffer to the suggested total—because extreme heat tends to mean ongoing sweating even after you stop moving.

Electrolytes: when they matter

Water is the base. But if you sweat a lot, you also lose electrolytes—especially sodium. That’s why people can feel “off” even after drinking plenty of water. If your session was long, very sweaty, or in heat, consider using a sports drink, a salty snack, or an electrolyte mix alongside water. You don’t need to overcomplicate it: for most people, “some sodium + water” is enough unless you’re an endurance athlete doing multiple hours.

The best simple check is your urine color and frequency later in the day. Pale yellow and regular bathroom breaks usually indicate you’re rehydrating well. If you’re very dark for hours, or you feel dizzy, treat that as a signal to slow down, cool down, and hydrate—and if symptoms are significant, seek medical advice.

FAQs

Is weight-loss always water?
Over a short session (like 30–120 minutes), most of the immediate scale change is water. Over days and weeks, it’s different (fat, glycogen, water). That’s why this calculator is meant for short windows like a workout, sauna, or a hot outdoor session.

Why can the calculator show 0% dehydration but still show sweat loss?
If you drank enough to offset most of what you lost, your net deficit is small even if you sweated a lot. That’s a win: you managed hydration during the session.

What if my post-weight is higher than my pre-weight?
That usually means you drank more than you lost, or the scale noise/clothing water skewed your numbers. In that case, the calculator will clamp deficit at 0 and treat sweat loss conservatively.

Should I “replace 150%” every time?
Not necessarily. If you’re lightly dehydrated and you’re not training again soon, 100% plus normal meals is fine. The higher range is more useful when you’ll exercise again within 6–12 hours or you’re in a hot environment.

Can I use this for illness or travel dehydration?
The weight method still works if your weight drop was mainly fluid (e.g., stomach bug, very dry flight), but it won’t capture everything. If you have symptoms like dizziness, confusion, or inability to keep fluids down, that can be serious—seek medical advice.

What’s a “dangerous” dehydration percentage?
People vary, but larger percentages generally correspond to bigger performance and heat-risk issues. If you’re feeling unwell, not urinating, or have heat illness symptoms, treat it as urgent rather than “just a number.”

Make it viral (how people actually share this)

• Gym buddy challenge: “Guess your sweat loss” then screenshot results.
• Sauna flex: compare sweat rate + rehydration plan (with electrolytes debate).
• Travel hack: run it after a flight and share “I was 1.2% dehydrated.”
• Hot-day PSA: post your plan as a story on a heatwave day.