Unit Conversion precision issue - sql

Question

Seeking an solution to the following problem.
In our application we were having an functionality of converting the units as per user selection. e.g . Convert kg to lbs or vice versa based on unit selection by user.
We have a screen where in user can select the unit in which he is providing the value e.g Kg or lbs
We have a table which contains all the formulas for the predefined unit which our system supports e.g 1Kg = 2.20462lbs like so
We fetch the formula from the database and calculate / evaluate the formula with actual values as provided by the user.
Since , user have ability to see any units as per his/her choice. we decided to save the value which user provided on screen in standard unit only in our database for easier maintainablity . in this case let's say we have decided to save in kg only database however , user may have selected 'lbs' on screen , but while saving we do convert the 'lbs' to 'kg' and then we will save to database
while reading from database we do convert to 'lbs' if user intended to see in 'lbs' on screen
Hope till now the implementaton is clear , now the problem to above solution is
Let's assume user have selected value '1' as 'lbs' and saved it
In database we will save in 'kg' so value of '1' is saved as 2.20462 in database
while reading again we will do convert to show in 'lbs' now we get value as 0.999998. this is expected due to precision in formula
But user asking to atleast show the same value what he as entered , in this case '1' but we have to do calculation to convert it back to 'lbs' since we are saving in 'kg' in database
I am seeking an solution to this problem what could be better solution with minimal changes.
Table Definition
Id | UnitConversion
1 | 100
2 | 200
3 | 30
Our application is developed on
Angularjs, webapi and sql server

Answer 1

Just for fun, here is a stripped down version of my conversion utility. Just showing MASS here. By storing the conversion factors as varchar, precision is at the individual record level.
Declare #Map table (MapType varchar(50),MapName varchar(50),MapValue varchar(50))
Insert Into #Map values
('Mass','tonnes (metric)','1000'),
('Mass','tons (US)' ,'907.18474'),
('Mass','tons (UK)' ,'1016.0469088'),
('Mass','stones' ,'6.35029318'),
('Mass','slugs(g-pounds)','14.593903'),
('Mass','Solar masses' ,'1.989e30'),
('Mass','pounds (troy)' ,'0.3732417216'),
('Mass','pounds' ,'0.45359237'),
('Mass','picograms' ,'1e-15'),
('Mass','ounces' ,'0.028349523'),
('Mass','ounces (troy)' ,'0.0311034768'),
('Mass','nanograms' ,'1e-12'),
('Mass','milligrams' ,'1e-6'),
('Mass','micrograms' ,'1e-9'),
('Mass','megatonnes' ,'1e9'),
('Mass','kilotonnes' ,'1e6'),
('Mass','kilograms' ,'1'), --- << Base
('Mass','hundredweights' ,'50.80234544'),
('Mass','hectograms' ,'0.1'),
('Mass','grams' ,'1e-3'),
('Mass','grains' ,'0.00006479891'),
('Mass','femtograms' ,'1e-18'),
('Mass','Earth masses' ,'5.980e24'),
('Mass','decagrams' ,'0.01'),
('Mass','cental' ,'45.359237'),
('Mass','carats (metric)','0.0002')
Declare #Value float = 1
Declare #From varchar(50)= 'kilograms'
Declare #To varchar(50)= 'pounds'
Select #Value * Max(IIF(MapName=#From,cast(MapValue as float),null)) / Max(IIF(MapName=#To,cast(MapValue as float),null))
From #Map
Where MapName in(#From,#To)
Returns
2.20462262184878

Answer 2

At Sql server side choose a finer scale then that you need to return to a client. For example, if you need 5 digits to the right of the decimal point, set scale to 7 for DB column
declare #n decimal(20,5) = 2.20462;
declare #t table (
m decimal(20,7)
);
insert #t(m) values
(1./#n),
(10./#n),
(1000./#n);
select cast(m*#n as decimal(20,5)) as r
from #t;
Note "Decimal and numeric are synonyms and can be used interchangeably." msdn.microsoft.com/en-us/library/ms187746.aspx