# Formula

In science, a **formula** is a concise way of expressing information symbolically, as in a mathematical formula or a chemical formula. The informal use of the term *formula* in science refers to the .

The plural of *formula* can be either *formulas* (from the most common English plural noun form) or, under the influence of scientific Latin, *formulae* (from the original Latin).^{[2]}

In mathematics, a formula generally refers to an identity which equates one mathematical expression to another, with the most important ones being mathematical theorems. Syntactically, a formula (often referred to as a *well-formed* formula) is an entity which is constructed using the symbols and formation rules of a given logical language.^{[3]} For example, determining the volume of a sphere requires a significant amount of integral calculus or its geometrical analogue, the method of exhaustion.^{[4]} However, having done this once in terms of some parameter (the radius for example), mathematicians have produced a formula to describe the volume of a sphere in terms of its radius:

Having obtained this result, the volume of any sphere can be computed as long as its radius is known. Here, notice that the volume *V* and the radius *r* are expressed as single letters instead of words or phrases. This convention, while less important in a relatively simple formula, means that mathematicians can more quickly manipulate formulas which are larger and more complex.^{[5]} Mathematical formulas are often algebraic, analytical or in closed form.^{[6]}

In modern chemistry, a chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using a single line of chemical element symbols, numbers, and sometimes other symbols, such as parentheses, brackets, and plus (+) and minus (−) signs.^{[7]} For example, H_{2}O is the chemical formula for water, specifying that each molecule consists of two hydrogen (H) atoms and one oxygen (O) atom. Similarly, O^{−}_{3} denotes an ozone molecule consisting of three oxygen atoms^{[8]} and a net negative charge.

In a general context, formulas are a manifestation of mathematical model to real world phenomena, and as such can be used to provide solution (or approximated solution) to real world problems, with some being more general than others. For example, the formula

is an expression of Newton's second law, and is applicable to a wide range of physical situations. Other formulas, such as the use of the equation of a sine curve to model the movement of the tides in a bay, may be created to solve a particular problem. In all cases, however, formulas form the basis for calculations.

Expressions are distinct from formulas in that they cannot contain an equals sign (=).^{[9]} Expressions can be likened to phrases the same way formulas can be likened to grammatical sentences.

A chemical formula identifies each constituent element by its chemical symbol, and indicates the proportionate number of atoms of each element.

In empirical formulas, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound—as ratios to the key element. For molecular compounds, these ratio numbers can always be expressed as whole numbers. For example, the empirical formula of ethanol may be written C_{2}H_{6}O,^{[10]} because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of ionic compounds, however, cannot be written as empirical formulas which contains only the whole numbers. An example is boron carbide, whose formula of CB_{n} is a variable non-whole number ratio, with n ranging from over 4 to more than 6.5.

When the chemical compound of the formula consists of simple molecules, chemical formulas often employ ways to suggest the structure of the molecule. There are several types of these formulas, including molecular formulas and condensed formulas. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the molecular formula for glucose is C_{6}H_{12}O_{6} rather than the glucose empirical formula, which is CH_{2}O. Except for the very simple substances, molecular chemical formulas generally lack needed structural information, and might even be ambiguous in occasions.

A structural formula is a drawing that shows the location of each atom, and which atoms it binds to.

In computing, a formula typically describes a calculation, such as addition, to be performed on one or more variables. A formula is often implicitly provided in the form of a computer instruction such as.

In computer spreadsheet software, a formula indicating how to compute the value of a cell, say *A3*, could be written as

where *A1* and *A2* refer to other cells (column A, row 1 or 2) within the spreadsheet. This is a shortcut for the "paper" form *A3 = A1+A2*, where *A3* is, by convention, omitted because the result is always stored in the cell itself, making the stating of the name redundant.

A physical quantity can be expressed as the product of a number and a physical unit, while a formula expresses a relationship between physical quantities. A necessary condition for a formula to be valid is the requirement that all terms have the same dimension, meaning that every term in the formula could be potentially converted to contain the identical unit (or product of identical units).^{[11]}

There is a vast amount of educational training about retaining units in computations, and converting units to a desirable form (such as the case of units conversion by factor-label).

In most likelihood, the vast majority of computations with measurements are done in computer programs, with no facility for retaining a symbolic computation of the units. Only the numerical quantity is used in the computation, which requires the universal formula to be converted to a formula intended to be used with prescribed units only (i.e., the numerical quantity is implicitly assumed to be multiplying a particular unit). The requirements about the prescribed units must be given to users of the input and the output of the formula.

The formula with prescribed units could also appear with simple symbols, perhaps even with identical symbols as in the original dimensional formula:

If the physical formula is not dimensionally homogeneous, it would be erroneous. In fact, the falsehood becomes apparent in the impossibility to derive a formula with prescribed units, as it would not be possible to derive a formula consisting only of numbers and dimensionless ratios.

Formulas used in science almost always require a choice of units.^{[14]} Formulas are used to express relationships between various quantities, such as temperature, mass, or charge in physics; supply, profit, or demand in economics; or a wide range of other quantities in other disciplines.

An example of a formula used in science is Boltzmann's entropy formula. In statistical thermodynamics, it is a probability equation relating the entropy *S* of an ideal gas to the quantity *W*, which is the number of microstates corresponding to a given macrostate:

where *k* is Boltzmann's constant equal to 1.38062 x 10^{−23} joule/kelvin, and *W* is the number of microstates consistent with the given macrostate.