- Weigh yourself on a calibrated scale every morning before eating, before drinking, and in light clothing. Use the same scale every day.
- Enter your dry weight — the weight your dialysis team has set as your target post-dialysis weight. If you do not know it, ask your nurse or nephrologist.
- Enter today's weight in kilograms.
- Select days since last dialysis — 1 day (morning after a session), 2 days (typical gap between Monday-Wednesday-Friday sessions), or 3 days (weekend gap).
- Select your approximate daily urine output. Anuric patients (no urine) have the strictest fluid limits.
- Press the button — you will see your weight gained, daily rate, estimated fluid allowance, and UFR at the next session, plus a color-coded action recommendation.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this tool daily — ideally every morning — if you are a hemodialysis patient who wants to know whether your current fluid gain is within safe limits, how much fluid you can still drink today, and what ultrafiltration rate to expect at your next session. It is most useful in the 1–3 days between dialysis treatments.
Appropriate population
Adult patients on maintenance hemodialysis (thrice-weekly or twice-weekly schedules) who weigh themselves on a calibrated scale every morning before eating and before their first drink. Also useful for peritoneal dialysis patients monitoring fluid balance, though the UFR estimate applies primarily to hemodialysis sessions. Require your established dry weight from your dialysis team — do not estimate it yourself.
When to call your dialysis center immediately
Do not wait for the calculator result if you have: shortness of breath at rest or lying flat, leg swelling that is new or rapidly worsening, blood pressure above 160/100 mmHg, or a weight gain of more than 3 kg since your last session. These are signs of acute fluid overload requiring urgent assessment — call your center or go to an emergency department now.
Pearls & Pitfalls
Salt is the real driver — not drinking too much water
Every gram of sodium consumed retains approximately 200 mL of water. A dialysis patient who eats 3 g of sodium during the weekend (two salty meals) will gain roughly 600 mL of retained fluid before they drink a single glass of water. Addressing sodium intake is more effective than counting milliliters of fluid.
The 3-day weekend gap is the highest-risk interval
Patients on a Monday-Wednesday-Friday schedule arrive on Monday having gone 3 days without dialysis. The weight gain over this interval is typically 50% higher than the 2-day gain. Planning stricter sodium and fluid control on Thursday–Sunday prevents the Monday crisis. Some centers offer a brief Friday afternoon session to limit the weekend gap for vulnerable patients.
Pitfalls
(1) Dry weight must be actively reassessed by your dialysis team — it changes with nutrition, muscle mass, and illness. Using an outdated dry weight makes every calculation inaccurate. (2) The UFR estimate assumes a standard 4-hour session. If your session is shorter, your true UFR is higher. Discuss session duration with your team when IDWG is consistently high. (3) Do not restrict fluids below the estimated allowance without medical guidance — dehydration in dialysis causes cramps, hypotension, and access clotting.
Why Use It
Interdialytic weight gain (IDWG) is the most directly controllable variable in dialysis management, and its consequences extend far beyond comfort. Every kilogram of excess fluid above dry weight imposes an estimated 13 mmHg rise in systolic blood pressure and requires rapid removal during the next dialysis session. The ultrafiltration rate (UFR) — the speed of fluid removal in mL per kg per hour — is independently associated with mortality when it exceeds 13 mL/kg/hr: at this rate, the cardiac chambers cannot refill fast enough during diastole, triggering intradialytic hypotension, myocardial ischemia, and arrhythmia. Chronic high UFR from repeated large IDWG is a leading cause of left ventricular dysfunction in dialysis patients. This calculator makes both the gain and the UFR visible so patients can act before the session, not after the damage is done.
Fluid & Weight Status Calculator
Enter your details below to calculate your current fluid gain status, safe daily fluid allowance, and how much fluid you need to limit today.
🧮 Interdialytic Weight & Fluid Calculator
For hemodialysis patients. Enter your weight and urine output to get personalized targets.
Weight Gained
Rate per Day
Daily Fluid Allowance
Ultrafiltration Rate Est.
⚠ This calculator is for educational guidance only. Your actual fluid allowance must be set by your nephrologist based on your clinical status, residual kidney function, blood pressure, and dialysis prescription. Never restrict fluids to the point of dehydration.
Next Steps
Use the result to support — not replace — clinical judgment.
- Interpret the value against the targets shown in the calculator and the Evidence section below, in the context of the full clinical picture.
- Trend serial measurements rather than acting on a single result; confirm abnormal or unexpected values before changing management.
- Apply the relevant KDIGO / specialty-guideline threshold and document the indication.
- Escalate or refer to nephrology when results are out of range, rapidly changing, or discordant with the clinical picture — and discuss the implications with the patient.
Evidence & References
Formula & Equations
| Quantity | Equation |
|---|---|
| Interdialytic Weight Gain (kg) | Today's weight − Dry weight |
| Rate per day (kg/day) | Weight gain ÷ Days since last dialysis |
| % gain of dry weight | (Weight gain ÷ Dry weight) × 100 |
| Daily fluid allowance (mL) | Urine output (mL/day) + 500 mL (insensible losses) |
| Ultrafiltration Rate (mL/kg/hr) | (Weight gain in kg × 1000) ÷ (Dry weight × 4 hours) |
Interpretation Thresholds
| Parameter | Safe / Target | Caution | Danger |
|---|---|---|---|
| IDWG per day | ≤1.0 kg/day | 1.0–2.0 kg/day | >2.0 kg/day |
| IDWG as % dry weight | ≤4.5% | 4.5–5.0% | >5.0% |
| UFR (4-hr session) | <10 mL/kg/hr | 10–13 mL/kg/hr | >13 mL/kg/hr |
The UFR formula assumes all accumulated fluid is removed in a 4-hour session. Session length varies; a longer session lowers the UFR. The insensible loss estimate of 500 mL/day is a clinical convention; actual losses vary with body size, fever, diarrhea, and ambient temperature (higher in tropical climates such as the Philippines).
Evidence & References
The 13 mL/kg/hr UFR threshold derives from prospective observational data in the DOPPS cohort and was formally incorporated into K/DOQI cardiovascular guidelines. The K/DOQI 2005 guideline established the framework for fluid and cardiovascular risk management in dialysis patients that remains foundational today. Subsequent data from Flythe et al. and Obi et al. confirmed that high UFR and the weekend inter-dialytic gap (3-day interval) are the strongest modifiable predictors of intradialytic hypotension and long-term cardiac mortality in hemodialysis.
- National Kidney Foundation. K/DOQI Clinical Practice Guidelines for Cardiovascular Disease in Dialysis Patients. American Journal of Kidney Diseases. 2005;45(4 Suppl 3):S1–S153.
