"similar quantity" is correct and usable in written English.
You can use it when you need to refer to a quantity that is comparable in size or number to something else. For example, "When we compared the two sets of data, we found that they had a similar quantity of elements."
In Hillsborough, Mr. Garland's nursery, Garland Truffles, supplies a similar quantity of inoculated trees.
Add a similar quantity of water to completely cover the broccoli.
Reliable sources assure me that yogurt, let down with a similar quantity of milk, will produce the same result.
A similar quantity of soyabeans has gone to Pakistan's poultry farms.
This article was amended on 6 March 2012 because the original said a similar quantity of seeds were sold at a Sotheby's auction last year.
Pre-lunch canisters of scotch were followed by a couple of glasses of wine during the meal and a similar quantity of post-meal cognac.
It was revealed that the transport potentials correspond to the driving force in nonequilibrium thermodynamics, and they contain a similar quantity, i.e. chemical potential divided by temperature.
Send one copy of any article, chapter, and other similar quantity of material by email, hard copy, or fax to one person at another location for that individual's personal use as above.
American forces consume more than 1m gallons of fuel a day in Afghanistan, and a similar quantity in Iraq.Until recently military planners had assumed that fuel would be plentiful and easily available.
One will collect ten or more kilos of refined Turkish or Afghan heroin for which the boss will pay £15,000-17,000 a kilo and perhaps a similar quantity of cocaine.Violence, like distribution, is outsourced.
It also discusses the system's potential for carbon sequestration and shows that a similar quantity of carbon is stored after 4 years under irrigated permanent beds to that after 11 21 years in rainfed no-tillage systems.
