Sodium deficit calculator
Estimate the sodium needed to bring serum sodium up to a target: a 70 kg man going from 120 to 140 mEq/L has about 42 litres of body water, so the deficit is 840 mEq. For learning, not for prescribing. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Sodium
- 120
- Target sodium
- 140
- Weight
- 70
- Sex
- Male
- Tbw fraction
- 0.6 mEq
- Tbw
- 42
- Delta sodium
- 20
- Sodium deficit
- 840
The sodium deficit calculator estimates how many milliequivalents of sodium would be needed to move serum sodium from a current laboratory value to a chosen target, using a sex-specific total-body-water fraction times body weight in kilograms. Clinicians in training, nursing students, and health-software teams use that identity when teaching hyponatremia replacement, reviewing electrolyte worksheets, or wiring educational calculators into apps. You send current serum sodium, target sodium, weight, and sex; the tool returns the deficit in milliequivalents together with the implied total body water and the sodium gap. Results are educational estimates only and do not replace serial laboratory monitoring, fluid choice, or professional medical advice.
What the sodium deficit formula measures and how total body water is estimated
The total body sodium deficit is the textbook teaching identity used when a serum sodium sits below a chosen target: milliequivalents of sodium theoretically needed to close that gap while holding total body water fixed. Sodium deficit equals total body water in litres times the difference between target sodium and current serum sodium, both in milliequivalents per litre, so the product is milliequivalents. Total body water is not measured here and is not the Watson equation; it is estimated as a sex-specific fraction of body weight in kilograms.
Adult male teaching default is zero point six times weight. Adult female teaching default is zero point five times weight. When current sodium is one hundred twenty milliequivalents per litre, the target is one hundred forty, and a seventy kilogram man is entered, total body water is forty-two litres and the deficit is eight hundred forty milliequivalents.
When current sodium already equals the target, the identity returns zero. When current sodium exceeds the target, the same identity returns a negative value, which means a surplus relative to the target rather than a replacement need. Current serum sodium that is not a strictly positive finite number is rejected: zero, negative, missing, empty, or non-numeric sodium is not a valid laboratory input for this formula.
The arithmetic is deterministic.
Who uses the deficit estimate, and its limits
Call the free in-browser sodium deficit calculator for classroom demos, nursing homework, and quick personal learning without reimplementing the sex-specific total-body-water identity in every spreadsheet. Keep requests small, pin the numeric fields in tests with the documented rounding, and you get reproducible figures for edge workers, LMS plugins, and consumer health funnels without standing up clinical decision support you are not licensed to offer.
Always disclose that the numbers are educational estimates: the fractions ignore age and body composition, the identity ignores ongoing urinary losses and potassium, and the result does not replace serial sodium checks, fluid-restriction rules, or clinician-prescribed correction limits. People with symptoms, known sodium disorders, kidney disease, heart failure, or medication changes should follow professional medical advice rather than any single automated number alone.
What you can do with it
Hyponatremia teaching worksheets
Education apps show how a seventy kilogram man at one hundred twenty milliequivalents per litre toward a target of one hundred forty yields eight hundred forty milliequivalents without recoding the fractions on every client.
FAQ
What happens if current serum sodium is zero or negative?
Current sodium that is not a positive number is not a valid laboratory input for this formula and is not charged.
Does this replace medical advice or use Watson total body water?
No. Results are educational estimates using adult teaching fractions 0.6 and 0.5, not the Watson equations. People with sodium disorders should follow professional guidance, not this calculator alone.
How is the sodium deficit calculated?
Total body water is estimated as 0.6 times weight in kilograms for men or 0.5 for women, then multiplied by the gap between target and current sodium. The result is in milliequivalents of sodium.
What does a negative sodium deficit mean?
That current sodium is already above the target. The same formula then shows a surplus relative to the target, not an amount to replace. If current and target sodium are equal, the deficit is zero.
What does this estimate leave out?
The water fractions ignore age, height and body composition, and the formula ignores ongoing urine losses and potassium. The number does not replace repeated sodium checks or the judgment of the clinical team.
Related tools
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/sodium-deficit \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"sodium":120,"target_sodium":140,"weight":70,"sex":"male"}'const res = await fetch("https://api.kit.forhosting.com/health/sodium-deficit", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"sodium": 120,
"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/sodium-deficit",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"sodium": 120,
"target_sodium": 140,
"weight": 70,
"sex": "male"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/sodium-deficit", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"sodium":120,"target_sodium":140,"weight":70,"sex":"male"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"sodium":120,"target_sodium":140,"weight":70,"sex":"male"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/sodium-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": 120,
"target_sodium": 140,
"weight": 70,
"sex": "male"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.sodium_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. |
Technical notes
Current sodium that is missing, zero, negative, or not a finite number fails with invalid_input and is not billed.
What the sodium deficit formula measures and how total body water is estimated
Those fractions ignore age, height, and adiposity on purpose so the endpoint stays a four-field calculator rather than a full body-composition model. Identical numbers always produce identical JSON, with stable rounding so tests can pin sodium_deficit across runtimes without floating-point surprises.
Input contract, response fields, and error behaviour
Target sodium must also be a strictly positive finite number inside the declared teaching range. Weight must be a finite positive number inside the declared teaching range because a non-positive mass cannot yield a usable total-body-water volume. Optional output keys are omitted rather than set to null. POST a JSON body with four required fields. Field sodium is current serum sodium in milliequivalents per litre; aliases include current_sodium, na, Na, serum_sodium, and sodium_meq. It must be a strictly positive finite number. Zero, negative, missing, empty, or non-numeric sodium fails with invalid_input naming the field and is not billed, because a laboratory sodium that is not a positive number is not a valid input for this formula. Field target_sodium is the desired serum sodium in the same units; aliases include desired_sodium, na_target, goal_sodium, and target_na. Field weight is body mass in kilograms; aliases include weight_kg, kg, mass, and w. Field sex selects the total-body-water fraction and is a closed enumeration: male, female, m, f, man, or woman, case-insensitive; alias gender. Anything outside that set fails with invalid_input. Each numeric field may arrive as a JSON number or as a numeric string. On success the body echoes the four inputs, reports tbw_fraction as zero point six or zero point five, reports tbw as that fraction times weight in litres, reports delta_sodium as target minus current, reports sodium_deficit as tbw times that delta in milliequivalents, sets unit to mEq, restates the formula string, and sets method to tbw_delta_na so clients can branch on algorithm identity. Pair this endpoint with serial sodium checks of your own; this calculator does not prescribe a correction rate or choose an intravenous fluid.
API usage, pricing, product workflows, and practical limits
Use the prepaid API when a health SaaS, electronic worksheet, simulation lab, tutoring platform, or school LMS needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Typical workflows send a hyponatremia panel with a male or female weight and a target of one hundred thirty-five or one hundred forty to show how many milliequivalents the textbook formula attributes to the gap, then pin sodium_deficit in automated checkers. Do not treat a large computed deficit as a bolus order; correction rate, fluid choice, and monitoring remain clinical decisions outside this endpoint.
Integration use cases
Nursing and medical-school homework
Students submit current sodium, target sodium, weight, and sex and pin sodium_deficit JSON in automated checkers with deterministic TBW-times-delta outputs.
Simulation labs and SaaS backends
Server pipelines that already store electrolyte panels, weight, and sex call a prepaid endpoint for sodium_deficit instead of duplicating the textbook identity in every language.
Technical questions
How is the sodium deficit calculated?
sodium_deficit = TBW × (target_sodium − sodium), with TBW = 0.6 × weight kg for males and 0.5 × weight kg for females. The result is milliequivalents (mEq). 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), target_sodium (desired Na in mEq/L), weight (kilograms), and sex (male or female). Missing fields return invalid_input and are not billed.
What does the sodium 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.