How to (Accurately) Measure Body Fat & Muscle Mass

Ranked from worst to best . . .

If you’ve ever searched:

• “What’s the most accurate way to measure body fat?”

• “Are smart scales accurate?”

• “DEXA vs InBody comparison”

• “Best way to track muscle gain for women”

You’re not alone.

Before you can set an attainable goal I strongly suggest gathering as much information about where you currently are. After all the best way to know where you are going, it to know where you are starting from.

Have you ever tried to give directions to someone when you don’t know where they are? You can’t tell them which way to go. Giving the same set of directions to 4 people starting in different places will always result in them getting lost, not reaching their destination.

Knowing your body fat percentage, lean muscle mass, and bone density both before you start a new workout regime, nutrition habit, or lifestyle habit can ensure you are headed in the best possible direction, as fast as possible. Ie. Taking the free way instead of backroads to get where you are going faster.

But here’s the thing:

Not all body fat measurement methods are created equal.

Some are highly influenced by hydration and hormones. Others are considered clinical gold standards. Understanding the margin of error helps you decide which tools are worth your attention — and which are just noise.

Below, we’ll break down the most common body composition methods, ranked from highest margin of error to lowest, so you can make informed decisions.

1. Visual Estimation & Progress Photos

Highest Margin of Error

This includes:

• Online body fat comparison charts

• Coaches “eyeballing” body fat percentage

• Mirror assessments

• Social media side-by-side photos

Why It’s So Inaccurate

Visual estimation is extremely subjective. Two women at 24–26% body fat can look completely different depending on:

• Muscle mass

• Fat distribution

• Genetics

• Lighting

• Pump from a workout

• Sodium intake

• Menstrual cycle phase

• Camera angles

Even professional trainers can be off by 5–10% when visually estimating body fat. While I do use this method often because of its’s low barrier to entry (hello FREE) it’s not always the best option especially when you have less fat to lose and are looking for more specific data.

When It’s Helpful

Progress photos taken under consistent conditions (same lighting, same time of day, same clothing) can show visual trends over time. Maybe not the most accurate for body fat %, but a picture is worth 1000 words.

But they should never be used to assign an exact body fat percentage.

Margin of error: Very high

Best used for: Visual comparison only.

2. Smart Scales (At-Home Bioelectrical Impedance Analysis – BIA)

Smart scales use bioelectrical impedance analysis (BIA). A small electrical current travels through your lower body to estimate fat mass and lean mass based on resistance. Fat tissue resists electrical flow more than muscle — so the scale estimates body composition using algorithms.

These are becoming very popular and widely available within a variety of price points, making them an attractive choice for someone who wants to monitor their progress daily or multiple times a week.

Why Smart Scales Fluctuate

Smart scales are heavily influenced by:

• Hydration levels

• Carbohydrate intake (glycogen binds water)

• Sodium intake

• Alcohol consumption

• Time of day

• Recent workouts

• Menstrual cycle changes

You can appear to “gain” several pounds of lean mass or body fat overnight due to water shifts alone. This can cause unnecessary panic for most, and have the exact opposite desired effect for adherence.

Most importantly: smart scales only measure the lower body and then estimate the rest. So if you’re cool with 50% of your data being a guess, this might be for you.

When They’re Useful

If used:

• First thing in the morning

• After using the restroom

• Before food or drink

• Under consistent hydration

They can provide trend data. They are not precise body fat measurement tools.

Margin of error: High

Best used for: Weekly trend tracking only.

3. Handheld Bioelectrical Impedance Devices

Handheld BIA devices work similarly to smart scales but measure upper-body impedance. Many gyms have these as they are less intimidating (and less expensive) than some other options.

Same science. Same limitations.

Limitations

• Highly hydration-sensitive

• Algorithm-based estimates

• Partial body measurement

• Large variability between devices

Because these rely on estimated equations, two devices can produce different readings on the same day.

Margin of error: High to moderate

Best used for: Rough consistency checks, not precision tracking.

In my opinion these are about as accurate as a visual guess from a qualified trainer. So you can take a shirtless selfie and probably get the same information. But that’s up to you.

4. Skinfold Calipers

Skinfold calipers measure subcutaneous fat thickness at multiple sites on the body (commonly 3, 7, or 9 locations). The measurements are entered into predictive equations to estimate total body fat percentage.

Not only is this uncomfortable for you, trust me - it’s uncomfortable for the trainer too! Being pinched and prodded 3 or 4 times to get the average reading is time consuming, invasive, and sometimes painful.

Advantages

• Affordable

• Portable

• More objective than visual estimation

• Good for tracking subcutaneous fat changes

Limitations

• Technician-dependent

• Does not measure visceral fat

• Less accurate for very lean or higher-BMI individuals

• Site measurement inconsistencies can skew data

When the same experienced technician performs the test each time, calipers can track trends reasonably well. But you need to see the same person every time.

But they are not perfect and can miss internal fat distribution, making them another undesirable choice IMO.

Margin of error: Moderate

Best used for: Consistent trend tracking with the same professional.

5. InBody Scans (Multi-Frequency BIA)

InBody machines use multi-frequency electrical currents through both hands and feet. This allows for segmental analysis of:

• Total body fat

• Skeletal muscle mass

• Lean mass by limb

• Estimated visceral fat

Why InBody Is More Advanced

• Multi-frequency measurement

• Segmental breakdown (arms, legs, trunk)

• More refined algorithms

Why It Still Has Error

Despite being more advanced than home scales, InBody scans are still influenced by:

• Hydration status

• Food intake

• Caffeine

• Exercise within 12–24 hours

• Menstrual cycle phase

Women can see 1–3% fluctuations purely from hormonal changes. Which doesn’t sound like much, but when you are < 20% body fat, the difference between 18% and 15% is pretty high.

If using InBody, standardize:

• Fasted state

• Same time of day

• No training beforehand

• Similar hydration status

Margin of error: Moderate to lower-moderate

Best used for: Quarterly progress tracking.

This is another one a lot of gyms are starting to use. Offering it to clients as a “perk” of coaching or training. I’ve never been very “salesy” but this always was a time gym management wanted us to push training, nutrition, or supplements onto people while we had them vulnerable on the scanner for a few minutes. Use it with a grain of salt, and stick to a trainer you trust who can help you interpret the results without being pushy.

6. Hydrostatic Weighing (Hydrodensitometry)

Hydrostatic weighing measures body density by calculating water displacement while fully submerged. Because fat is less dense than water and muscle is denser, body composition can be estimated from body density. If you have your swim cap handy, this one may be for you!

Advantages

• Historically considered a gold standard

• Less influenced by hydration than BIA

• Scientifically validated

Limitations

• Requires full exhale underwater

• Air trapped in lungs affects results

• Limited availability

• Uncomfortable for some individuals

Accuracy depends heavily on proper technique.

Margin of error: Low (when performed correctly)

Best used for: Clinical-level body fat testing where available.

Personally, this is one I have never done, as I have yet to find a convenient location with availability that works for me. If you don’t mind a dunk tank and a hunt, I would go find one and try it! But just in case that sounds like too much of an inconvenience, I’ve listed my last and favorite option below.

7. DEXA Scan (Dual-Energy X-ray Absorptiometry)

Lowest Margin of Error

DEXA scan uses low-dose X-rays to differentiate between:

• Fat mass

• Lean mass

• Bone mineral density

• Visceral fat

Why DEXA Is Considered the Most Accurate

• Direct tissue differentiation

• Regional breakdown of body composition

• Measures visceral fat

• Tracks bone density (critical for women 35+)

• Highly repeatable when standardized

For women focused on strength training, long-term metabolic health, and bone density preservation, DEXA provides the most comprehensive data.

Limitations

• Higher cost

• Small radiation exposure

• Slight variability between machines

Even DEXA has a margin of error — but it is the lowest available in non-research settings.

Margin of error: Lowest

Best used for: Annual comprehensive body composition and bone health assessment.

This test takes <10 minutes, is more widely available than hyrdostatic weighing with a tremendous degree of accuracy. Often locations sell packages making them significantly more affordable, and encouraging you to come back for an update. I like every 6 months, but yearly would be great too to stretch your cost.

How to Choose the Right Body Composition Method

For most women:

• Smart scale → Weekly trend awareness

• InBody → Quarterly check-in

• DEXA → Annual precision assessment

But here’s what matters more than any number:

• Progressive overload in your training

• Strength increases

• Waist measurements

• Energy levels

• Sleep quality

• Hormonal health

• Sustainability

Body fat percentage is one data point — not your identity.

Local Recommendation: DexaFit Gig Harbor

If you’re in the South Sound area and want the most accurate body composition testing available, I recommend:

DexaFit Gig Harbor

They offer:

• DEXA body composition scans

• Bone density analysis

• Visceral fat measurement

• Resting metabolic rate testing

I went in last week and the owner walked me through the process, made me feel immediately at ease, and interpreted the results with me directly after the scan. I was in and out in under 30 minutes with a full analysis of my body fat, muscularity, bone density, even my biological age (turns out I’m younger than you all thought!) It helped me set some good tangible goals for myself for the coming 6 months or training and nutrition. As a bonus, all the data is stored in an app so I can easily compare each of my scans in the future to track my progress.

If you book, use my code: Essential15 for a discount on services.

Because if you’re going to collect data — make sure it’s quality data. That’s the way we do less, better.