Standard weight of the English penny

year	weight in grains		dollar equivalent (1834-1873)		Remark
8th century to approx 991	24.	00	$0.	0645	traditional weight
approx 991 to approx 1275	22.	50	$0.	0605	traditional weight
about 1275	22.	00	$0.	0591
1343	20.	30	0.	0546
1345	20.	15	0.	0542
1346	20.	00	0.	0538
1351	18.	00	0.	0484
1412	15.	00	0.	0403
1464	12.	00	0.	0323
1526	10.	70	0.	0288
1544	10.	00	0.	0269
1552	8.	00	0.	0215	(11 oz 1 dwt fineness)
1560	8.	00	0.	0215	(sterling fineness)
1601	7.	80	0.	0210
1816	7.	27	0.	0195

Source: Feavearyear, Appendix 3-2, p. 439

Notes

English silver coins usually circulated at a premium above the bullion value of their silver content.

Weights of medieval coins were haphazard. Coiners would take a troy pound of silver and cut it up into bits of reasonably equal size, but it was not required (nor, in those days, technically possible) to make them exactly the same size.

Until the adoption of the milled (reeded) coin edge in the 17th Century made detection easy, "clipping" (shaving) the edge off bullion coins was a rampant problem. "There were periods when every coin in circulation had been clipped down to little more than one-half its proper weight." [Feavearyear, p. 6]

Between clipping and counterfeiting, the value of English silver coinage was not nearly as stable as the table above would imply. Periodically, kings would recall the old coinage, usually paying out no more than its bullion value, or sometimes repudiate it, leaving holders to make their own sales for bullion value. The mints would issue new coins at full weight. Before long, the new coins would again be "clipped" down.

The "great debasement" of the silver coinage under Henry VIII and Edward VI does not show up in the table above, because it was reversed by Elizabeth I ca. 1560.

As early as 1553, Mestrell demonstrated how to reed coin edges, but, running into jealousy from established English coiners, he was forced from the mint in 1572 and even hanged (on counterfeiting charges) in 1578. In 1639, Pierre Blondeau demonstrated a successful machine to France's Louis XIII, and Louis "banished hammered coins from his mint forever" in 1645. After many years of obstruction, Blondeau was finally allowed to mint English coins with reeded edges in 1663.

Blondeau's full-weight coins could not be clipped, but they could be, and were, melted down, while old underweight clipped coins continued to be used in daily commerce. It was not until Secretary of the Treasury William Lowndes recalled the old coinage in 1696 that underweight clipped coins finally disappeared, and full-weight milled coins became standard in circulation.

* * *
The term "penny," in its earliest form "pending," may refer to a coin issued by Penda, King of Mercia, in the second quarter of the 7th Century. Its continuous history started later, however, with coins struck by Mercia's King Offa in 760 CE.

The internationally respected "sterling" (92.5% pure) standard for silver coinage became established by William the Conqueror's time, or even earlier [Feavearyear, p. 8], though the name itself (of unknown origin, originally a term for "penny") did not appear until the early 12th Century.

Note on 1834 silver dollar equivalents used here:
For medieval times, silver was valued highly against local labor and agricultural produce, but cheaply against imports. Examples:


In Anglo-Saxon times, an ox was priced at 30 silver pennies, $1.935 in 1834 weight, or $20-$40 in 2003 currency, cheap in modern terms.
(Ox price given in Dorothy Whitelock, The Pelican History of England Vol 2: The Beginnings of English Society: The Anglo-Saxon Period, Penguin Books, 1952; p. 9)

"In 1176, [King] Henry II imported 34 pounds of sugar for his own use at the rate of 9 pence a pound." Using the table above, that would be $0.5445 a pound, or $5-$10 per pound in 2003 currency, expensive in modern terms.
Quote from Doris Mary Stenton, The Pelican History of England Volume 3: English Society in the Early Middle Ages (1066-1307), Penguin Books, 1951; p. 28.