Everyone talks about growers and showers. It's all over Reddit, locker room conversations, and internet forums. But only one clinical study has ever formally measured and defined the difference. Yafi et al. (2018) measured 274 men and established the first real data-based definition. Here's what they found — and why it matters more than you think.
Published in the International Journal of Impotence Research, this study out of Tulane University measured 274 men using penile duplex Doppler ultrasound. Each man was measured flaccid, then again at peak erection after injection of a vasodilating agent (prostaglandin E1). This isn't self-reporting — these are clinical measurements.
The median change in penile length from flaccid to erect was 4.0 cm (1.6 inches). The researchers used that median as their cutoff:
Gained ≥4 cm (1.6 in) from flaccid to erect
Average gain: 5.3 cm (2.1 in)
Gained <4 cm (1.6 in) from flaccid to erect
Average gain: 3.1 cm (1.2 in)
Key finding: The only statistically significant predictor of being a grower was younger age. Growers averaged 47.5 years old vs. 55.9 for showers. No other factor — height, weight, BMI, ethnicity, vascular status — was a significant predictor.
Important caveat: All 274 participants in the Yafi study were being evaluated for erectile dysfunction. This means the sample skews older and may not perfectly represent the general population. Men with ED may show less flaccid-to-erect variation than healthy men. That said, it remains the only study to formally define and measure the grower/shower distinction.
The landmark Veale et al. (2015) systematic review — which pooled data from 15,521 men measured by health professionals — gives us the population averages:
The average man goes from 3.61 inches flaccid to 5.16 inches erect — a gain of about 1.55 inches, or roughly 43%. But the variation around that average is enormous. Some men barely change at all. Others nearly double in length.
One of the most clinically useful findings from Veale 2015: stretched flaccid length (5.21 inches) is almost identical to erect length (5.16 inches). This is why urologists use stretched length as a proxy when measuring — it predicts erect size far better than resting flaccid length does.
Your flaccid penis changes size constantly throughout the day. It's not a fixed measurement — it's a snapshot of your body's current state. Factors that shrink or expand your flaccid size:
Cold triggers the cremaster muscle to retract the testicles and the smooth muscle in the penis to contract, reducing blood volume. This can cut flaccid size by 50% or more. A hot shower does the opposite. (The full science of shrinkage)
The sympathetic nervous system (fight-or-flight) diverts blood away from the genitals toward muscles and organs. Anxiety, nervousness, or physical exertion all cause temporary shrinkage.
Post-workout shrinkage is real. Your body redirects blood to working muscles. Combine that with sweat-induced cooling, and you get maximum retraction. This is why locker room comparisons are completely useless.
The Yafi study found that younger men were more likely to be growers. This tracks with basic physiology — younger tissue has greater elasticity and vascular responsiveness. As men age, smooth muscle tone changes, and the gap between flaccid and erect size narrows. Not because erections get bigger, but because flaccid size stays closer to erect size.
Testosterone peaks in the morning and dips in the afternoon. Blood flow, body temperature, and hydration levels fluctuate constantly. Your penis at 7 AM and your penis at 6 PM after a stressful day are not the same size — and both are normal.
False. Veale 2015 found only a weak correlation between flaccid and erect length. The man who looks biggest in the locker room may be the same size as you when erect — or smaller. Flaccid size just tells you someone's current blood flow and tissue resting state. That's it.
Actually, the Yafi data says the opposite: 74% were showers. But this depends entirely on how you define the threshold. Using 4 cm as the cutoff, most men show moderate — not dramatic — change. The internet loves the "grower" narrative because it's reassuring, but the data says most men fall into the shower category.
Your position on the spectrum shifts throughout your life. Younger men tend to be more "grower" (greater elasticity, bigger flaccid-to-erect change). As vascular health changes with age, the gap tends to narrow. Weight, fitness, medications, and overall health all influence where you fall at any given time.
Being a shower says nothing about erect size. A shower simply starts closer to their erect size when flaccid. A grower with a 2-inch flaccid penis that doubles to 6 inches erect is larger than a shower with a 4.5-inch flaccid that reaches 5 inches. The categories describe change ratio, not final size.
The grower/shower distinction matters for exactly one reason: it proves that comparing flaccid penises is completely pointless.
If you've ever felt inadequate in a locker room, at a urinal, or just looking down at yourself — this data is for you. The guy next to you who looks bigger flaccid might be the exact same size as you when it counts. Or smaller. You literally cannot know from looking.
And if you're a grower who's been anxious about your flaccid size, the data is on your side: 26% of men experience the same dramatic transformation you do. You're not unusual. You're not broken. You're a quarter of the male population.
Your flaccid size is a roulette wheel. Your erect size is what matters — and it's probably more normal than you think.
Use the CalculatorThe grower vs. shower spectrum is real, measurable, and clinically documented. But it's a spectrum, not a binary. Most men (74%) show moderate change. A quarter show dramatic change. Younger men lean grower. The only thing it doesn't predict is what actually matters — your erect size.
If someone tells you they can gauge erect size from looking at a flaccid penis, they're wrong. The Veale data — 15,521 men — confirms that flaccid length is a poor predictor of erect length. Two men who look identical flaccid can differ by inches when erect. And two men who look radically different soft can be the same size hard.
Stop comparing the unreliable measurement. Check the one that matters.
Sources: Yafi et al. (2018), "Grower or shower? Predictors of change in penile length from the flaccid to erect state," International Journal of Impotence Research, 30(6), 287–291. Veale et al. (2015), "Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15,521 men," BJU International, 115(6), 978–986.
Disclaimer: We are not doctors. This article summarizes publicly available peer-reviewed research. It is not medical advice. See a board-certified urologist for personalized guidance.