Peptide Dosing by Body Weight: The Complete Calculation Guide

Why Body Weight Matters for Peptide Dosing

In clinical pharmacology, drug dosing can follow one of two approaches: flat dosing (everyone gets the same amount) or weight-based dosing (the dose scales with body mass). The choice between them depends on how a drug is distributed and metabolized in the body.

For peptides, the question is particularly relevant because most preclinical research expresses doses in mcg/kg or mg/kg — explicitly weight-dependent units. When these doses are translated to human use, the weight-based framework often gets lost, replaced by simplified flat-dose recommendations that may not serve everyone equally well.

This guide explains when weight-based dosing matters, how to perform the calculations, and which peptides use which approach. If you are new to peptide dosing generally, start with our Peptide Dosage Guide.

Flat Dose vs Weight-Based Dose

Flat Dosing

Flat dosing means every person receives the same milligram or microgram amount regardless of body weight. This approach works when:

• The drug has a wide therapeutic window (large gap between effective dose and toxic dose)
• The drug acts on a specific receptor system that saturates at a relatively consistent concentration
• Pharmacokinetic variability between individuals is primarily driven by factors other than weight (genetics, organ function)

Many FDA-approved peptide medications use flat dosing with titration. Semaglutide, for example, starts everyone at 0.25 mg weekly and titrates up regardless of body weight — though heavier patients may ultimately need higher maintenance doses for equivalent efficacy.

Weight-Based Dosing

Weight-based dosing scales the amount administered according to body mass. This approach is used when:

• The drug distributes into total body water or lean body mass proportionally
• Clearance rates correlate with body size
• The therapeutic window is narrower, making precise dosing more important
• Clinical trial data supports improved outcomes with weight-based protocols

Allometric Scaling: From Animal Data to Human Doses

Most peptides in the research community do not have large human clinical trial datasets. Dosing is often extrapolated from animal studies, which requires understanding allometric scaling — the mathematical relationship between body size and physiological processes across species.

Why You Cannot Just Convert by Weight

A common mistake is to take a rat dose (e.g., 10 mcg/kg) and directly apply it to a human. This dramatically overestimates the human dose because smaller animals have faster metabolic rates per unit of body weight. A rat metabolizes drugs roughly 6-7 times faster per kilogram than a human.

The FDA Body Surface Area Method

The FDA recommends converting animal doses to human equivalent doses (HED) using body surface area (BSA) ratios:^[1]

Species	Conversion Factor (to Human)
Mouse	x 0.081
Rat	x 0.162
Rabbit	x 0.324
Dog	x 0.541
Monkey	x 0.324

Formula: Human dose (mcg/kg) = Animal dose (mcg/kg) x Conversion factor

Then multiply by body weight in kg to get total dose in mcg.

Worked Example: BPC-157

BPC-157 is typically studied at 10 mcg/kg in rats:

• Human equivalent = 10 x 0.162 = 1.62 mcg/kg
• For a 70 kg person: 1.62 x 70 = 113 mcg
• For a 100 kg person: 1.62 x 100 = 162 mcg

The commonly cited range of 200-500 mcg includes a safety margin above the minimum HED, accounting for individual variability. See our BPC-157 Dosage Guide for the complete dosing protocol.

Common Peptides: Which Dosing Approach Applies?

Peptide	Dosing Approach	Typical Range	Notes
Semaglutide	Flat + titration	0.25-2.4 mg/week	FDA-approved; titrate by response
Tirzepatide	Flat + titration	2.5-15 mg/week	FDA-approved; titrate by response
BPC-157	Weight-based	200-500 mcg/day	Scaled from 10 mcg/kg rat dose
TB-500	Flat (typically)	2-2.5 mg 2x/week	Standard flat protocol; less weight-sensitive
Ipamorelin	Flat or mild scaling	200-300 mcg/injection	Some practitioners scale modestly by weight
CJC-1295	Flat or mild scaling	100-300 mcg/injection	DAC version: 2 mg/week flat; no-DAC: weight-variable
Tesamorelin	Flat	2 mg/day	FDA-approved flat dose for visceral fat
Sermorelin	Weight-based	2-3 mcg/kg	Original clinical dosing was weight-based
AOD-9604	Flat	300 mcg/day	Clinical trial used flat dosing
GHK-Cu	Flat (topical/injection)	1-2 mg topical; 100-200 mcg SQ	Route-dependent dosing

How to Calculate a Weight-Based Peptide Dose

Step 1: Identify the mcg/kg Target

Use the allometrically scaled human equivalent dose, not the raw animal dose. Common scaled ranges:

• BPC-157: 1.6-8 mcg/kg HED (from 10-50 mcg/kg rat)
• Sermorelin: 2-3 mcg/kg (from original clinical protocols)
• GHRP-6: 1-2 mcg/kg HED (from 5-10 mcg/kg rat)

Step 2: Multiply by Body Weight

Total dose (mcg) = Target (mcg/kg) x Body weight (kg)

Examples for BPC-157 at ~3 mcg/kg HED (middle of the range):

• 60 kg person: 3 x 60 = 180 mcg per dose
• 80 kg person: 3 x 80 = 240 mcg per dose
• 100 kg person: 3 x 100 = 300 mcg per dose
• 120 kg person: 3 x 120 = 360 mcg per dose

Step 3: Round to Practical Syringe Units

Insulin syringes measure in 1-unit (0.01 mL) increments. After reconstitution, round your calculated volume to the nearest whole unit. Use our Reconstitution Calculator to convert mcg doses to exact syringe volumes based on your vial concentration.

Should You Dose by Total Weight or Lean Body Mass?

For most peptides, total body weight is used for calculation because it is simple to measure. However, lean body mass (LBM) may be more pharmacologically relevant in some cases:

• Hydrophilic peptides (most peptides) distribute primarily in lean tissue and water. A person with 40% body fat has a very different distribution volume than someone at 15% body fat at the same total weight
• GH secretagogues (CJC-1295, Ipamorelin, Sermorelin): IGF-1 response correlates better with lean mass than total weight. An extremely overweight individual may not need proportionally more GH secretagogue
• GLP-1 agonists: Dose titration is based on clinical response, not weight. Higher body weight does correlate with needing higher doses for equivalent weight loss percentage, but this is managed through titration, not weight-based calculation

Practical rule of thumb: If your body fat percentage is under 30%, total body weight is a reasonable proxy. Above 30%, consider using an adjusted body weight formula:

Adjusted weight = Lean body mass + (0.4 x Fat mass)

When Body Weight Does Not Matter

Several scenarios make weight-based dosing unnecessary:

• FDA-approved medications with established titration schedules: Semaglutide, tirzepatide, and tesamorelin have well-defined protocols that account for individual variation through titration rather than weight calculation
• Topical peptides: GHK-Cu creams and serums are applied to the skin surface. The relevant variable is the area of application, not body weight
• Very wide therapeutic window: When a peptide is effective across a 5-10x dose range with minimal side effects at the high end, precise weight-based dosing adds complexity without meaningful benefit

Practical Decision Framework

Use this framework to decide whether to scale by weight:

1. Is there an FDA-approved protocol? If yes, follow the approved titration schedule
2. Is the peptide derived from animal research with mcg/kg dosing? If yes, allometric scaling is appropriate
3. Are you significantly above or below 80 kg? If you are 60-90 kg, standard flat-dose ranges are probably fine. Outside that range, weight-based adjustment adds value
4. Is the therapeutic window narrow? If side effects occur close to the effective dose, weight-based precision matters more

For compound-specific dosing, visit the individual peptide pages in our Compound Database or use the Dosage Calculator tool. For reconstitution calculations, use the Reconstitution Calculator.