What does eGFR measure?

eGFR (estimated Glomerular Filtration Rate) measures how well your kidneys filter waste from the blood, based on a blood creatinine test. An eGFR of 60 roughly means your kidneys are functioning at about 60% of normal capacity.

What is a normal eGFR?

An eGFR of 90 or above is generally considered normal kidney function. Values between 60-89 indicate mildly decreased function, and values consistently below 60 for more than three months are used to help diagnose chronic kidney disease (CKD).

How does a sleep cycle calculator work?

Sleep cycle calculators work backward or forward from a target time using the typical 90-minute sleep cycle length, aiming to wake you up at the end of a complete cycle rather than mid-cycle, which can reduce grogginess on waking.

Health & Fitness 📅 2026-06-19 ⏱ 9 min read

GFR, Sleep & Body Surface Area: Three Health Numbers Worth Knowing

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MegaCalcOnline Health Team

Evidence-based health calculators · Updated 2026-06-19

What eGFR (kidney function) actually measures and why it matters, how sleep cycle calculators time wake-ups around natural sleep stages, and what body surface area is used for.

What eGFR actually measures
CKD stages explained
How sleep cycle calculators work
Body surface area — what it's used for

What eGFR Actually Measures

eGFR stands for estimated Glomerular Filtration Rate — a measure of how well your kidneys are filtering waste products out of your blood. It's calculated from a routine blood test measuring creatinine (a waste product your muscles produce), combined with age, sex, and sometimes ethnicity, using a standard formula.

According to Kidney Health Australia, your eGFR is roughly equivalent to the percentage of normal kidney function you have — an eGFR of 60 broadly means your kidneys are working at about 60% of full capacity. In Australia, eGFR is automatically calculated and reported on blood test results for anyone aged 18 and over.

🧮 Calculate Your eGFR

Estimate your kidney function using your recent blood test creatinine result.

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CKD Stages Explained

Stage	eGFR Range	Description
G1	90+	Normal kidney function (if other damage markers present)
G2	60-89	Mildly decreased function
G3a	45-59	Mild to moderate decrease
G3b	30-44	Moderate to severe decrease
G4	15-29	Severely decreased function
G5	Below 15	Kidney failure

⚠️ A single low reading isn't a diagnosis. Chronic Kidney Disease (CKD) is generally only diagnosed when reduced eGFR persists for more than three months, often alongside other markers like protein in the urine (albuminuria). A single test result should always be discussed with your GP or a nephrologist, not interpreted in isolation.

Your GP monitors eGFR over time as part of routine care, since a declining trend matters more than any single number — this is why your doctor will typically compare results against your previous tests rather than judging one reading alone.

How Sleep Cycle Calculators Work

Sleep generally moves through cycles of roughly 90 minutes each, progressing through lighter and deeper sleep stages including REM (rapid eye movement) sleep. Waking up in the middle of a deep sleep stage tends to produce more grogginess (sometimes called sleep inertia) than waking at the natural end of a complete cycle.

Sleep cycle calculators work backward from a wake-up time (suggesting bedtimes that align with full 90-minute cycles) or forward from a bedtime (suggesting wake times at the end of complete cycles), rather than simply recommending a flat "8 hours" regardless of timing.

💡 Worth knowing: 90 minutes is a population average, not a precise figure for every individual — actual cycle length varies somewhat between people and even night to night. The calculator gives a useful starting point for experimentation rather than an exact science for your specific body.

🧮 Calculate Your Sleep Cycles

Find bedtimes or wake times that align with complete 90-minute sleep cycles.

Open Sleep Calculator →

Body Surface Area — What It's Used For

Body Surface Area (BSA) estimates the total surface area of your skin, calculated from height and weight using formulas like the Du Bois formula. While less commonly discussed than BMI or body fat percentage, BSA has genuine clinical importance:

Medication dosing: Some medications, particularly chemotherapy drugs, are dosed based on BSA rather than weight alone, since it correlates well with blood volume and metabolic rate
Burn assessment: Medical professionals use BSA to estimate the percentage of skin affected by burns, which guides treatment decisions
General research reference: BSA is also used as a standardisation reference in some other health calculations, including the eGFR formula covered above (results are standardised to a reference BSA of 1.73 m²)

🧮 Calculate Your Body Surface Area

Get your estimated BSA using the standard Du Bois formula.

Open Body Surface Area Calculator →

⚠️ General Information Only: This article provides general health and fitness information for educational purposes. It is not medical advice and should not replace consultation with a doctor, dietitian, or other qualified health professional. Individual results and needs vary significantly based on personal health history, medications and other factors.