Free water deficit calculator
The free water deficit calculator estimates how many liters of electrolyte-free water would be required, in a teaching identity, to lower serum sodium from a current high laboratory value to a chosen target while holding estimated total body water as a sex-specific fraction of body weight.
Run — free
Educators, nursing students, and product teams use that identity when teaching hypernatremia, checking electrolyte worksheets, or embedding a small educational calculator. Send current serum sodium, target sodium, weight in kilograms, and sex; the response reports the deficit in liters together with total body water and the fraction used. If current sodium is below the target, the request fails because the identity describes a water deficit of hypernatremia, not a sodium deficit of hyponatremia. Weight that is not a positive number also fails. Validation errors are not billed. The free browser widget and the API share one pure module, and successful API calls cost $0.002. Figures are educational estimates and never replace serial sodium checks, fluid choice, or professional medical advice.
How to use it
Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.
Check your inputs
Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.
Use it again or automate it
Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.
What you can do with it
Get an answer now
Enter one set of values and see the result without building a spreadsheet or script.
Compare scenarios
Change one value at a time and rerun the calculation to understand what affects the result.
Automate repeated work
Use the API when the same calculation needs to run inside your product or workflow.
FAQ
How is the free water deficit calculated?
water_deficit = TBW × (sodium / target_sodium − 1), with TBW = 0.6 × weight kg for males and 0.5 × weight kg for females. The result is litres (L). The response also returns tbw, tbw_fraction, and delta_sodium.
What fields are required?
Four required fields: sodium (current serum Na in mEq/L, must be a positive number and at least the target), target_sodium (desired Na in mEq/L), weight (kilograms, must be a positive number), and sex (male or female). Missing fields return invalid_input and are not billed.
What happens if current serum sodium is below the target?
The request fails with invalid_input. Free water deficit is the hypernatremia teaching identity; a current sodium below the target is a different problem and is not charged.
What does the free water deficit calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.
Does this replace medical advice or choose a correction rate?
No. Results are educational estimates using adult teaching fractions 0.6 and 0.5, not a fluid order. People with sodium disorders should follow professional guidance, not this calculator alone.
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/health/water-deficit \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"sodium":160,"target_sodium":140,"weight":70,"sex":"male"}'const res = await fetch("https://api.kit.forhosting.com/health/water-deficit", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"sodium": 160,
"target_sodium": 140,
"weight": 70,
"sex": "male"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/water-deficit",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"sodium": 160,
"target_sodium": 140,
"weight": 70,
"sex": "male"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/water-deficit", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"sodium":160,"target_sodium":140,"weight":70,"sex":"male"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"sodium":160,"target_sodium":140,"weight":70,"sex":"male"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/water-deficit", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"sodium": 160,
"target_sodium": 140,
"weight": 70,
"sex": "male"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.water_deficit",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |