Intake and Output (I&O)

Intake and output (I&O) monitoring is a fundamental nursing responsibility. Tracking how much fluid goes into and out of a patient helps the healthcare team assess kidney function, hydration status, cardiac function, and the effectiveness of treatments like diuretics or IV fluid therapy. The math is straightforward addition and subtraction — but accuracy matters, because clinical decisions depend on these numbers.

What Counts as Intake

Intake is all fluid that enters the patient’s body, measured in milliliters (mL).

Bladder Irrigation Note: When continuous bladder irrigation (CBI) is running, the irrigant is instilled into the bladder and drains back out through the catheter. To calculate true urine output, subtract the total volume of irrigant instilled from the total catheter drainage:

$\text{True Urine Output} = \text{Total Drainage} - \text{Irrigant Instilled}$

For example, if 2,000 mL of irrigant was instilled and total catheter drainage was 2,350 mL, the true urine output is $2{,}350 - 2{,}000 = 350$ mL. The irrigant itself is not counted as intake. If total drainage is less than the volume instilled, this indicates retained fluid and should be reported to the provider, as it may signal catheter obstruction or other complications.

Important: Ice chips melt to approximately half their volume. This is a standard conversion used in clinical practice:

$\text{Ice chip intake (mL)} = \text{Volume of ice chips (mL)} \times 0.5$

What Counts as Output

Output is all fluid that leaves the patient’s body, also measured in mL.

Calculating Fluid Balance

Fluid balance is the difference between total intake and total output over a given time period (usually one shift or 24 hours):

$\text{Fluid Balance} = \text{Total Intake} - \text{Total Output}$

• Positive fluid balance means the patient took in more than they put out (fluid retention)
• Negative fluid balance means the patient put out more than they took in (fluid loss)

Worked Examples

Example 1: Simple Shift Calculation

During a 12-hour day shift, a patient has the following:

Intake:

• Breakfast: 240 mL juice, 180 mL coffee
• IV: Normal Saline running at 125 mL/hr for 12 hours
• Lunch: 200 mL water, 120 mL soup broth
• Snack: 150 mL of ice chips

Output:

• Urine: 350 mL at 0800, 425 mL at 1200, 300 mL at 1600
• Emesis: 150 mL at 1000

Step 1: Calculate total intake.

Oral fluids: $240 + 180 + 200 + 120 = 740$ mL

Ice chips: $150 \times 0.5 = 75$ mL

IV fluids: $125 \text{ mL/hr} \times 12 \text{ hr} = 1{,}500$ mL

$\text{Total Intake} = 740 + 75 + 1{,}500 = 2{,}315 \text{ mL}$

Step 2: Calculate total output.

Urine: $350 + 425 + 300 = 1{,}075$ mL

Emesis: $150$ mL

$\text{Total Output} = 1{,}075 + 150 = 1{,}225 \text{ mL}$

Step 3: Calculate fluid balance.

$\text{Fluid Balance} = 2{,}315 - 1{,}225 = +1{,}090 \text{ mL}$

Answer: The patient has a positive fluid balance of 1,090 mL for this shift.

Example 2: 24-Hour I&O with Multiple Sources

A post-surgical patient has the following over 24 hours:

Intake:

• IV Lactated Ringer’s: 100 mL/hr for 24 hours = 2,400 mL
• IVPB Cefazolin in 100 mL NS q8h = $100 \times 3 = 300$ mL
• Oral fluids: 150 mL + 200 mL + 120 mL = 470 mL

Output:

• Foley catheter: 1,850 mL over 24 hours
• Jackson-Pratt (JP) drain: 85 mL + 70 mL = 155 mL
• Emesis: 200 mL

$\text{Total Intake} = 2{,}400 + 300 + 470 = 3{,}170 \text{ mL}$

$\text{Total Output} = 1{,}850 + 155 + 200 = 2{,}205 \text{ mL}$

$\text{Fluid Balance} = 3{,}170 - 2{,}205 = +965 \text{ mL}$

Answer: Positive fluid balance of 965 mL over 24 hours.

Example 3: Negative Fluid Balance

A patient receiving Furosemide 40 mg IV BID (a diuretic) has the following during a 12-hour shift:

Intake: IV D5W at 50 mL/hr for 12 hours = 600 mL; oral fluids = 360 mL. Total intake = 960 mL.

Output: Urine = 1,450 mL. Total output = 1,450 mL.

$\text{Fluid Balance} = 960 - 1{,}450 = -490 \text{ mL}$

Answer: Negative fluid balance of 490 mL — expected with diuretic therapy. The medication is working as intended.

Sample I&O Record

Fluid balance: $2{,}510 - 1{,}270 = +1{,}240$ mL

When to Report Imbalances

Alert the provider when you observe:

• Urine output below 30 mL/hr (or below 0.5 mL/kg/hr) for two or more consecutive hours — may indicate renal impairment or dehydration
• Large positive fluid balance (over 1,000 mL per shift) in patients with heart failure or renal disease — risk of fluid overload
• Large negative fluid balance not explained by diuretic therapy — may indicate dehydration or hemorrhage
• Sudden increase in drainage from surgical drains — may indicate internal bleeding
• No urine output (anuria) — report immediately

Practice Problems

Test your understanding with these problems. Click to reveal each answer.

Common Mistakes to Avoid

1. Forgetting to halve ice chips. Recording the full volume of ice chips instead of 50% is one of the most common I&O errors.
2. Missing IV piggyback volumes. IVPB antibiotics and other medications delivered in small bags (50–250 mL) are intake and must be included.
3. Not recording flush volumes. Saline flushes used before and after IV medications add up over a shift. Per INS (Infusion Nurses Society) standards, typical flush volumes are 3-5 mL for peripheral lines and 10 mL for central lines.
4. Miscounting bladder irrigation. Irrigation fluid is not recorded as intake. Instead, subtract the instilled irrigant volume from total catheter drainage to calculate true urine output (see the Bladder Irrigation Note above). If you instill 500 mL and 650 mL drains out, the true urine output is 150 mL. If drainage is less than the instilled volume, report the deficit to the provider.
5. Forgetting to convert household measures. If a patient drinks “a cup” of water, that is 240 mL — not an unmeasured amount. Use the standard conversions.

Key Takeaways

• Intake includes all oral fluids, IV fluids, tube feedings, blood products, and ice chips (at 50%)
• Output includes urine, emesis, drainage, diarrhea, NG suction, and blood loss
• Fluid balance = Total Intake $-$ Total Output; positive means retention, negative means loss
• Report urine output below 0.5 mL/kg/hr, unexplained large positive or negative balances, or sudden changes in drainage
• Accurate I&O tracking depends on measuring (not estimating) whenever possible, converting household volumes to mL, and documenting every source

Math for Nurses